We publish a nominal damage threshold for most of our thermal BB sensors as 20KW/cm2. Other manufacturers may quote higher numbers than this. In actuality, in one to one tests against competitors, our sensors show a higher damage threshold but the actual damage threshold depends on the total power as well as the power density. For very low powers such as 30W, the damage threshold can be as high as 50KW/cm2 and at high powers such as 5KW, it drops to 3KW/cm2. The Ophir sensor finder program takes account of these variations in its calculations.
Power Meters FAQ's
The damage threshold of thermal sensors does depend on the power level and not only the power density because the sensor disc itself gets hotter at high powers. For instance, the damage threshold of the Ophir broadband coating may be 50KW/cm2 at 10 Watts but only 10KW/cm2 at 300W. The Ophir specifications for damage threshold are always given for the highest power of use of a particular sensor, something which is not done by most other manufacturers. This should be taken into account when comparing specifications. The Sensor Finder takes the power level into consideration when calculating damage threshold.
The damage threshold curve in the sensors catalog only goes down to 1ns but the energy damage threshold is similar for shorter pulses. You can use ½ of the ns value for fs pulses i.e. the absorber damages twice as easily.
Corrosion is caused by interactions between the metallic components of the sensor and the cooling water, which may contain a variety of dissolved ions. Many factors affect the risk of corrosion forming, but the most important are the pH and the mixture of ions in the water. For this reason, we recommend using neutral deionized water in a closed circulating system (pH between 6 and 8). Please note that deionized water is usually slightly acidic (pH 5.65) due to absorption of CO2 from the atmosphere. The cooling water can be neutralized by adding 5 ml of a 10 mM solution of NaOH for each liter of water in the cooling system.
To prevent corrosion it is also crucial to not allow standing water to evaporate inside the sensor when it is not in use. When disconnecting a sensor from the cooling system, the water channel should be cleared by blowing compressed air through it.
The commercial additive Optishield Plus is also recommended for systems such as ours that have copper and aluminum in them. It has the additional benefit of having biocides to prevent buildup of organic contamination.
For those customers still experiencing problems with corrosion, we recommend the new thermal sensor 1000WP-BB-34 which has a special design in which all materials that come into contact with the cooling water are either copper or nonmetallic.
After the Helios reaches the maximum temperature of 60° C (approx. 40 kJ of accumulated energy), it should take about 10 20 minutes for it to cool back down to room temperature. Therfore, use the temperature sensor as the indication of how many pulses can be measured
Whether RS232 or Profinet is used, there is a command to query the current temperature. The customer is responsible for integrating this into the measurement script and coordinating with the laser control to make sure the laser is not allowed to be measured when the temperature is over the limit. If using the PC application, one should select: Options > Log Temperature Enable. This will show the current temperature (and log it). If the temperature goes over the limit, it will turn red.
An example of this is the 10K-W, which uses a reflective cone to spread the beam before it reaches the absorber. Because of the way the 10K-W is built, a small beam in the center is spread out more than a large beam. A 10mm beam, for example, is spread out to about 5 x140mm = 7cm² a reduction in power density of 9:1 . A 45mm beam is spread out to about 22.5 x 140mm = 31cm². The power density of the 10mm beam is reduced 9 times, but the power density of the 45mm beam only goes down by about 2 times. This does not apply to sensors that don’t have a cone reflector.
For the Ariel, we do not specify the “damage threshold” as such, and instead we give the table of recommended beam size and exposure time (near the bottom of the specifications page). This is the best way to define the operating parameters since the damage threshold is dependent on the exposure time for these types of measurements (where we are measuring powers that are much higher than what the sensor can measure in CW mode). So, the best way to avoid damage is to follow the specified guidelines for exposure times and beam sizes.