Absorption Capabilities

Absorption is the tiny fraction of laser light an optical component converts into heat. Measured in parts per million (ppm), it is easy to overlook, yet it drives thermal lensing, focal shift, beam distortion, and coating failure in high-power laser systems.

Every ppm counts: the less a coating absorbs, the less it heats up and the more stable the laser system stays. OPTOMAN’s dense ion beam sputtered (IBS) coatings, made from ultra-high-purity materials, keep absorption to a minimum – so the beam stays unchanged at full power, hour after hour.

ABSORPTION MEASUREMENTS AND REPORTS

Wavelength, nm	Substrate material	Measurement wavelength, nm	AOI, deg	Polarization	L-scan max, ppm	Time scan, ppm	T-scan avg, ppm
343	BBO	355	2	P	486	450	163
980-1080	UVFS	1070	2	P	1.32	1.38	1.13
1070	IRFS	1070	2	P	0.123	0.1	0.113
1055-1080	UVFS	1070	2	P	1.25	0.85	0.9
1064	UVFS	1070	2	P	1.6	1.75	1.635
1065-1075	IRFS	1070	2	P	0.091	0.1	0.076

Wavelength, nm	Substrate material	Measurement wavelength, nm	Measurement wavelength, nm	Polarization	L-scan max, ppm	Time scan, ppm	T-scan avg, ppm
337-349	UVFS	355	45	S	95	90	82
343	UVFS	355	45	S	51.3	50	44.4
980-1080	UVFS	1070	2	P	16.05	16.2	16.21
1010-1050	UVFS	1064	45	P	0.5–1.2	0.5–1	0.5–1
1030 + 630-660	UVFS	1064	0	P	0.4	0.6	0.5
1030 + 650-660	UVFS	1070	45	S	0.745	0.75	0.735
1030 + 1060-1100	UVFS	1070	45	P	0.74	0.8	0.61
1030 + 1064	UVFS	1070	2	P	3.57	3.65	3.672

Wavelength, nm	Substrate material	Measurement wavelength, nm	AOI, deg	Polarization	L-scan max, ppm	Time scan, ppm	Time scan, ppm
355 + 532 +1064	UVFS	355	45	S	76.7	85	85
970-990 + 1015-1045	UVFS	1070	2	P	1.52	1.62	1.619
1035-1075 + 1550 + 915 + 808	UVFS	1070	45	P	1.6	1.4	1.57

Wavelength, nm	Substrate material	Measurement wavelength, nm	AOI, deg	Polarization	L-scan max, ppm	Time scan, ppm	T-scan avg, ppm
1015-1045	UVFS	1070	60	P	4.12	4	4.04
1020-1040	UVFS	1070	49	S	2.1	2.2	2.16
1020-1040	UVFS	1070	49	P	8.7	8.6	8.7

HOW ABSORPTION DEPENDS ON WAVELENGTH

Between the near-infrared around 1 µm and its third harmonic in the UV, absorption differs by orders of magnitude – the shorter the wavelength, the higher the photon energy and the closer it comes to the band gap of the coating materials.

In the ultraviolet, photon energy approaches the band gap of common high-index oxides, and impurities, defects and band-tail states start to absorb. UV light can also create new absorbing defects over time, so stability under exposure matters as much as the initial value. That is why UV coatings rely on wide-band gap materials and extremely clean deposition.

The near-infrared is where dielectric coatings perform best. Photon energy is far below the band gap, so intrinsic absorption is negligible and only impurities and defects remain. Dense, amorphous IBS layers made from ultra-high-purity materials keep these to a minimum – critical for multi-kilowatt CW and high-average-power ultrafast lasers, where the absorbed heat grows in direct proportion to laser power.

The type of optic matters too. In an HR mirror, light reaches only the top layers of the coating, while in AR coatings and the transmitted polarization of a polarizer it passes through the whole stack. For femtosecond and picosecond lasers, high peak intensity adds nonlinear absorption – strongest in the UV.

Disclaimer

Absorption values were measured by LIDARIS using photothermal common-path interferometry (PCI), following the same standardized procedure for every sample, on a single coated surface. In PCI, a pump laser at the test wavelength heats the coating slightly, and a low-power probe beam detects the resulting thermal lens. The signal is proportional to the absorbed power, so absorption can be resolved down to sub-ppm levels.

Each sample is evaluated with three complementary scans:

Longitudinal scan (L-scan) – the measurement point is moved through the thickness of the optic, from the front surface to the rear surface. Absorption shows up as sharp peaks at each surface, while a raised baseline between them indicates bulk absorption in the substrate. This separates coating absorption from substrate absorption.

Time scan – the beams are held on one spot of the coated surface for about 11 minutes. This shows whether absorption is stable under continuous irradiation, or whether it drifts as the coating heats up or conditions.

Transverse scan (T-scan) – the measurement point is moved 3 mm across the coated surface. This shows how uniform absorption is across the coating and reveals any local defects or contamination. The reported absorption value is the median of this scan. The uncertainty is the spread of values with statistical outliers removed, so a single defect does not distort the result.