R+T = 1 - losses
Practically the reflection for coatings with low losses through absorption and scattering is R = 1-T.
For 1064 nm / 0° a standard EBE-coating has absorption < 20 ppm and scattering < 150 ppm. Lower losses (< 10ppm) can be achieved with IBS coatings (see Extremely low loss laser optics).
For mirrors it is easier to measure the transmission than reflection with standard spectrophotometers. For this reason we normally provide transmission curves for mirrors.
Wavelength shift with changing AOI
With increasing angle of incidence (AOI) the central wavelength of a dielectric coating shifts to shorter wavelengths.
The example shows a standard HR 1064 nm /0°.
It shifts about 10% when using AOI = 45° but only less than 1% when using AOI < 13°.
HR – polarization
Dielectric mirrors have a broader reflection band and higher reflection for s-polarized light compared to p-polarized light.
The example shows a standard HR 1064 nm / 45° that will achieve:
Rs > 99.9%
Ru > 99.8%
Rp > 99.7%
HR – Phase
In the reflected phase a standard mirror has a shift of 180°. By a simple variation of the top layer thickness the phase falls down to zero.
The example shows the reflected phase of a mirror HR 1064 nm / 0°
a) standard design
b) phase optimized
If necessary every coating can be optimized for its phase specification.
Reflected phase of different coating designs HR 1064 nm / 0°
HR – GDD
(Group Delay Dispersion)
Standard mirrors have already a low GDD but only at the center wavelength as shown here for standard mirror HR 1064 nm / 45°.
Especially for fs-laser systems the GDD becomes important. LASEROPTIK can supply GDD optimized optics to your specifications.
For more information see chapter Dispersive coatings
(Laser Induced Damage Threshold)
The LIDT of a coating strongly depends on coating materials and production methods.
Normally coatings with the highest LIDT will not have the highest reflection.
The example shows LIDT (pulsed laser) and reflection for different mirror types HR (0°) made with different coating materials (EBE).
The peaks in LIDT result from the water band at 940 nm, can be minimized by using IAD or sputtered coatings.
The LIDT also depends on the repetition rate and pulse duration τ.
Rules of thumb exist for pulsed laser systems:
1) LIDT ~ 2-lg (rep-rate)
2) LIDT ~ τ1/2 (τ > 0.1 ns)
LIDT ~ τ1/3 (τ < 20 ps)
Going down from ns-pulses to ps-pulses the damage mechanisms change and the prominent square root rule will not fit anymore.