Pulsed lasers are as popular as continuous wave (CW) lasers. The physics of many lasers lends itself to pulsed operation, and just as important, so do many applications. Pulse widths are becoming very short, reaching femto-seconds. When concentrating the laser energy into such short pulses and onto a small spot, even low pulse energy - in the μJ range - is sufficient to melt, cut, ablate, and weld materials without the effect of slowly heating and damaging adjacent areas. While pulsed lasers exist in many wavelengths from UV to IR, UV lasers have some unique applications in material processing and medicine, due to the high absorption of UV in many materials and the high photon energy at short wavelengths. All the aforementioned advantages of pulsed lasers, and in particular pulsed UV lasers, become challenges in the design and measurement of these lasers. For example, optical components inside a UV pulsed laser must perform reliably over thousands of hours of exposure to this radiation.
In the field of metrology, CW and pulsed lasers are measured with different instruments. CW laser power is measured using a photodiode or a thermopile sensor while pulse energy of repetitively pulsed lasers is measured using a pyroelectric sensor. The heat generated by the absorption of a laser pulse in the pyroelectric crystal produces a transient measurable voltage (proportional to the absorbed energy) across the crystal. When the heat dissipates, the voltage vanishes and the crystal is ready for the next pulse. However, the same properties that make short laser pulses very efficient in affecting change in various materials, pose a challenge for measuring devices - which by definition are required to be stable and provide a trusted measurement. Again, when working with UV lasers, this challenge becomes more pronounced.
Ophir has several types of absorber coatings that are used with pyroelectric crystals; these coatings also protect the crystals from being damaged. When the pulse energy density becomes too high even for these absorbers, diffusers are often used to spread the beam and reduce the energy density on the absorber. Ophir sensors with diffusers have the letters ‘DIF’ or ‘DIFH’ in the model name, for example, ‘PE50-DIF-C’. While the diffusers do a good job in spreading the beam, they also cannot escape the challenges the UV radiation introduces. When exposed to UV, organic contaminants that accumulate on the diffusers are carbonized. This reduces the transmission of the diffusers and changes the reading of the sensor by a few percent. Further exposure to UV light ablates the carbonized contaminants and the transmission recovers to its original value. After a while, more contaminants may accumulate on the diffuser and the process starts over. Exposing the diffuser to UV for several minutes before the actual measurement can ensure a correct measurement, but this isn’t always possible or convenient. Luckily, there is a solution:
Ophir offers two types of diffusers for its energy sensors: ‘DIF’ – Sensors with this "standard" diffuser are calibrated for wavelengths ranging from 355nm to 2100nm. ‘DIFH’ – Ophir offers two sensors with a special type of diffuser that has both a higher damage threshold and a special surface microstructure that prevents the accumulation of contamination. The PE50U-DIFH-C and PE50BF-DIFH-C are Ophir’s premium energy sensors. They are calibrated at wavelengths starting from 193nm all the way to 2940nm, and have a particularly high damage threshold even at these UV wavelengths (1 J/cm2 at 193nm). These 2 sensors are the best solution for pulsed lasers at 193nm and 248nm.