Radiation Absorbers

• Tight needs for laser absorbers

In all kind of detectors, a highly resistant absorber is a prominent ingredient that contributes to ensure their cor- rect operation, performance and reliability.
On thermopile detectors the absorbing coating is directly deposited on the same substrate where the thermocou- ples also lay, while in calorimeters the absorber coats the water cooled elements used as heat exchangers.
When designing a new device for laser power measurements, there are tight parameters for the materials that must be investigated and to which the material, then selected as laser absorber, must comply.
First, a number of chemical, physical and structural parameters that influence its damage threshold capability must be evaluated and tested.
Those parameters vary from absorber to absorber and from manufacturer to manufacturer.
Damage threshold is defined as the power density (W/cm2) beyond which it is encountered a variation >1% in the measurement of laser power, mostly as a consequence of an irreversible change in the
chemical and physical properties of the materials after laser absorption.
Among those parameters, both the melting points and thermal conductivities (W/m *°K) of materials must be carefully considered and must be the highest possible.
Materials must also maintain a constant behaviour on variations of temperature and, above all, resist without degrading or detaching from substrate upfront to ex- treme thermal stresses: those can be very high as it happens in the case of narrow Gaussian beams or in the case of localized delivery of laser radiation (hot spots). Thermal dimensioning and material selection can be said to be really optimized when the area inter- ested by the laser is kept below 250°C, even with sev- eral KW of laser power applied. 

In the case of pulsed lasers, also pulse duration has a sound influence on the damage threshold and can drive to substantially two modalities of coating damage. The dam- age process is ablative for very short pulses ( below 100 nsec): in this temporal regime the diffusion time of generated heat within the material is much longer than the pulse length itself and this condi- tion entails a strong localization of laser energy and the direct abla- tion of the absorber’s atoms. On the other extreme, with a pulse duration sufficiently long to allow a diffusion of heat within the ab-sorber (pulses > 10msec), damages are created by thermal effect. 
The other important parameter to be considered for materials is their absorption coefficient in the laser wavelength ranges, which needs to have the following general characteristics:
-be as high as possible (typically >70%), to guarantee an efficient absorption of radiation even in the case of very thin thick-ness of deposited materials and to provide the lowest reflection at any wavelength;
-have a spectral response that covers the broadest range of laser wavelengths;-
-provide the lowest possible reflection at any incidence angle;
The above general behaviors must last over time and possibly, (misuse is not considered) for the entire lifetime of the instru-ment; measurements, in fact, must not be affected by ageing or any change of properties (like oxidations) which might modify the chemical and optical properties of the absorbing surface.
Given the above constraints, manufacturers of laser measurement instrumentation are compelled to make very selective tech-no- logical choices because many of those materials, which could be potential candidates to be absorbers for the tough envi-ron- ments of high powers and high brightness lasers, have one or more characteristics that do not comply with those of a suitable coating.
A successful technological achievement, reached by combining new technologies, materials and by overcoming the con-straints described in the present article is Laserpoint’s Super Hard Coating (SHC).
Its property to allow an efficient and fast heat transfer gives it the capacity to resist to extremely high power densities and has been the real engine that generated two latest instruments designed for high power lasers.
 

• Surface Absorbers

Surface absorbers consist of a thin layers of materials, in general made of special mattes or refractory materials, deposited onto substrates, that can easily transfer heat like high conductivity metals. They are used for CW lasers or other sources that emit long pulses (with duration >300μsec). Radiation is almost entirely absorbed within that thin layer and then released as heat that flows through the thermopile.


 

• Volume Absorbers

In lasers that deliver short time pulses ( lower than microseconds), the time needed by heat to flow away from the impact area and to be removed by the cooling system is longer than the duration of the pulse length. An excess of heat remains con-centrated within a thin layer on the sensor’s surface where it generates a sudden overheating of the absorbing material. Above a certain level, this excess of energy can cause damages; often it will cause the absorber’s ablation.
To overcome similar situations, volume absorption technology is used. Volume absorption is where a gradual absorption of radiation occurs as this penetrates into the material. Total absorption is obtained on depths of 0.5-2mm rather than on few microns: the consequence is a slower distribution of energy and a far lower local temperature increase.
Various types of glasses and ceramics are used by Laser Point to cover the UV-C range (190-250nm) , the UV-A (250-400nm) and the VIS-NIR (BB absorber from 400nm to 5μm).Those absorbers can withstand peak powers of 100GW/cm2 and energy densities up to 30J/cm2.


 

• The very best of laser absorbers: the SHC coating

Laser Point’s SHC coating is a real high power coating. Performances of SHC are stunning and definitely place it to be the best laser coating available on the market.
Specification curves and values for the SHC are based on test campaigns made with our customers in disrup-tive conditions and show that the properties of SHC are steps ahead any other coating: it withstands more than 12KW/ cm2 in CW operation, with an effective applied power of 1KW of Yag laser, or 40J/cm2 with laser diodes peak powers of 3.2 KW @1 msec!
 

Compared to other absorbers for high powers that can be used on limited spectral ranges, the SHC also has an extended working range ( 0,25μ to 11μ) and a very high absorption ratio, making it suitable for a safe use in almost all laser applications. The graph shows a comparison of damage threshold values among Laserpoint’s SHC absorber against various types of commercial high power laser absorbers.
The upper graph shows the SHC behavior under different laser pulses conditions up to CW.
The lower graph shows the results obtained when a 6KW head from Laser Point coated with SHC has been compared, under multi-kilowatt beams, against similar heads from our Competi-tors bearing their best coating.