Radiation Absorbers

LaserPoint detectors can measure over the complete range of wavelengths interested by lasers, up to many kilowatts of power and hundreds of Joules of energy. It is thus evident that there is a necessity to use many kinds of different absorbers to accommodate so many situations.

A - Surface Absorbers

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

Surface Absorber:

Continuous Emission or Long Pulses (>500µsec) Light penetrates within a thin layer of optically absorbing refractory materials (10-40 µm thick) Generated heat flows through substrate to thermocouples and cooling system.

Short Pulses (<500µsec) Light penetrates within the same thin layer of optically absorbing material, as previously shown. During the short laser pulse duration, a sudden amount of heat is generated but it cannot flow through substrate. Major damages to absorbers may occur. Volume Absorbers offer far better performances.

Broadband Coatings

For low powers and general purpose applications LaserPoint utilizes a black coating (BBF) derived from astronomical research. The power density capability is limited to 200W/cm2, but has the important advantages of a flat spectral response in the range 0.19-25µm and the bearing of a very high absorptivity (> 96%) over the entire range.

A general purpose hard coating (HCB) for more demanding applications is used by LaserPoint for mid power lasers. It can be used over a broad spectral band (0.25-11µm) and some sensors ( as the W-200) bear specially shaped surfaces that reach a 150% absorptivity by dumping the radiation with multiple bounces. This coating reaches 2500W/cm2 at full power with 300W CW operation. At pulsed operations it can withstand up to 300J with long (msec) pulses.

High Power Coatings

Standard broadband absorbers can hardly withstand the disruptive situations generated by high power industrial lasers. LaserPoint has developed new high density and better thermal conductivity coatings for those demanding lasers. On LaserPoint heads from 1KW to 3KW the HPB absorbers can withstand 4KW/cm2 for CO2 in CW operations. LaserPoint’s HPI coating was developed for even more stringent situations and is applied on the 6KW head; it withstands 5KW/cm2 at 10.6µm and works in the range 0.8-1.5µm-10.6µm.

SHC coating

The SHC is a real high power coatings. Performances of SHC are stunning and definitely place it to be the best laser coating available on the market. Laser Point’s specification curves for the SHC are based on test campaigns made with our customers in disruptive 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 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 which can be used on limited spectral ranges, SHC has also a very high absorption ratio ( > 85%) over an extended working range ( 0,25µ to 25µ), making it suitable for safe applications in almost all laser domains.

Excimer Coatings

LaserPoint has developed a very hard absorber with flat response in the range 190-400µm, ideal for its use with industrial Excimer lasers. It has also a remarkable high absorptivity at 10.6µm, so it may as well be used for CO2 lasers. The ridged substrate surface induces multiple reflections that increase absorption to the highest available levels on the market. In the UV the coating can withstand >0.3J/ cm2, high repetition rate, nsec pulses at 248nm and has a peak power density capability >20KW/cm2.

B - Volume Absorbers

When heads with surface absorbers are used to measure lasers which deliver short time pulses ( lower than microseconds), heat has no time to flow and to be removed within the duration of the pulse length. Radiation remains deposited within in a thin surface layer where it generates a sudden overheating of the absorbing material and where the excess of energy will often cause ablation.
In those cases the technology of volume absorption is used, where a gradual, exponential decrease of radiation intensity occurs as it penetrates into the material. Total absorptions are obtained over 0.5-2mm depths rather than few microns: the consequence is a better distribution of energy and a far lower local temperature increase.
Various types of glasses and ceramics are used by LaserPoint to cover the UV-C range (190-250nm) , the UV-A (250-400nm) and the VIS-NIR (BB absorber from 400nm to 3µm). Those absorbers can withstand peak powers of 100GW/cm2 and energy densities up to 30J/cm2.

Volume Absorber

Short Pulses (<500 µsec) Light penetrates and is absorbed by a a thin layer of gradually absorbing refractory materials (1-2 mm thick). Heat is generated within a volume and safety flow through substrate to thermocouples and cooling system. Volume absorbers can measure very short pulses and high energies much better than surface absorbers.

Coating Damages

The damage threshold is defined as the level at which readings will change more than 1%. When exceeding this value the absorber surfaces will visibly change colour and the head calibration has to be reconsidered if the damaged area is important compared to the overall. In some cases a complete substitution of the sensor may be necessary. Yet, as very often happens, there may be a bleach on the coating but no at all, or just negligible change in the readings: those colour changes are barely aesthetical and can be accepted without other interventions on the head.