HR standards

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.

Transmission and reflection of a standard HR 1064 nm /0°.

Transmission and reflection of a standard HR 1064 nm /0°.

HR – bandwidth

Different coating material combinations have different bandwidths. Not every combination is suitable for every wavelength or application.

The transmission of four different types of coating material  combinations.

The transmission of four different types of coating material combinations.

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°.

Wavelength shift with changing AOI of a standard HR 1064 nm /0°

Wavelength shift with changing AOI of a standard HR 1064 nm /0°

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° will achieve:
Rs > 99.9%
Ru > 99.8%
Rp > 99.7% 

Reflection of a standard HR 1064 nm /45° with different polarizations.

Reflection of a standard HR 1064 nm /45° with different polarizations.

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°

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.

GDD of a standard mirror HR 1064 nm /45°

GDD of a standard mirror HR 1064 nm /45°

LIDT
(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.

LIDT and reflection shown for different mirror types HR (0°) made with different coating materials (
LIDT and reflection shown for different mirror types HR (0°) made with different coating materials (

LIDT and reflection shown for different mirror types HR (0°) made with different coating materials (EBE).

The LIDT also depends on the repetition rate and pulse width.

Two rules of thumb exist for pulsed laser systems:

1)  LIDT ~ 2-lg (rep-rate)    
2)  LIDT ~ √_________________________
2)  LIDT ~ √ pulse duration

Going down from ns-pulses to ps-pulses the square root rule will not fit anymore, the damage mechanisms change.