Thermopile laser sensor

[7] Pyroelectric sensor can measure low to medium energies (mJ to J) and are prone to microphonic effects.

[7] Calorimeters are capable of measuring high energies (mJ to kJ) but have large response times.

When a laser beam hits the surface of a thermopile sensor, the incident radiation is absorbed within the coating layer and transformed into heat.

[9] Due to the thermoelectric effect, the temperature difference causes an electrical voltage to build up within each thermocouple.

If a laser with pulse length in the range of 10−7 – 10−4 sec is used the sensor can be damaged by either dielectric break-down or thermal effects.

[12] In case of thermal damage, heat is deposited in a short time and cannot be dissipated until the next pulse arrives.

[11] For dielectric breakdown, the peak energy density during a pulse is high enough to locally ionize the sensor surface.

[13] To protect the sensor from damages by short optical pulses, volume absorbers are used with absorption lengths in the order of millimetres.

Thermopile discs have thermocouples deposited onto an aluminium plate in a radial arrangement as shown in Fig 3(a).

[8] The absorption coating in the illuminated area converts radiation into heat which flows radially outwards generating a temperature gradient between inner and outer ring and thus a thermoelectric voltage.

[8] Fig 3(b) shows the cross sectional view of the axial sensor where the temperature difference is established between the top and bottom surfaces.

Thermocouples are embedded into a matrix and aligned parallel with respect to the heat flow, forming junctions at top and bottom.

For constant incident laser power a larger absorption coefficient means more heat is generated[16] leading to increase in output voltage.

The background error can be minimized by keeping the sensor at ambient temperature and avoiding convective air flows.

According to EU standard (EN6001-1-22), every medical laser system needs to be equipped with a redundant power measurement unit.

For procedures such as precise tissue cutting and ablation the laser power can be measured before operation or even continuously throughout the process.

Fig 7 illustrates the option of mounting the detector behind the back mirror of a laser cavity for continuous monitoring.

The signal is recorded and processed in a read-out unit which displays the measured laser power (Fig 8).

Figure 1: [ 1 ] Thermal sensors are available in various sizes
Figure 2: [ 8 ] Working principle of a thermal laser sensor (Adapted from figure 3 with permission)
Figure 3: [ 8 ] (a) Radial Thermopile and (b) Axial Thermopile Sensors
Figure 4: [ 14 ] Axial sensor with 0.5 mm thickness
Figure 5: [ 8 ] Rise time comparison between Radial and axial thermopile sensors
Figure 6: [ 21 ] An example showing how thermal sensors can be used for continuous measurement
Figure 7: [ 21 ] An example showing how the thermal sensors can be used for continuous monitoring using back mirror
Figure 8: [ 22 ] Thorlab's thermal power meter
Figure 9: [ 23 ] Position sensor, with different quadrant as shown in the image