IR and Pulsed Laser Measurements

Many lasers, especially sealed CO2 lasers, use pulse width modulation (PWM) to control the power level of the laser. This is not true, pulsed operation, but rather a reduction of the duty cycle to lower the average power. The beam operates as if it were CW, and many operators do not even realize that the laser is pulsing. However, when attempting to measure a PWM laser with a scanning slit profiler, it must be treated as a pulsed laser source. To use the pulsed mode of the NanoScan the laser's pulse frequency should be at least several kHz, and the combination of the frequency and beam size must provide a sufficient number of pulses across the beam to generate a meaningful profile. Ten to fifteen pulses are a reasonable minimum. PWM lasers usually operate around 10kHz. The relationship of the beam size and frequency is a fairly simple mathematical model. The NanoScan drum speed is software controlled from 1.25Hz to 20Hz. There are two available drum sizes for the NanoScan; the standard head has a drum diameter of 42mm and the large aperture and high power heads use a larger drum with 84mm diameter. On the 42mm drum at the 1.25Hz rotation rate the slits travel at around 116.6mm per second or 116.6µm per millisecond. At a 10kHz laser repetition rate, a 175µm beam would have 15 pulses during the time that the slit was traversing it. A smaller beam would require a faster pulse rate, a larger one could perhaps run at a lower repetition rate. For example, a 1.0mm beam could be measured with a pulse rate as low as 2kHz and still provide a profile. There is a table of minimum beam sizes and pulse frequencies for the large and small hubs and scan speeds at the end of this document. It is recommended that the 1.25Hz scan speed be used for pulsed beams, however, if the beam sizes are large enough, or the pulse rates fast enough, the measurement can be sped up by increasing the scan speed to 2.5Hz or above. The NanoScan software will generate a warning if the scan rate is set too high for the pulse rate or beam size. This warning algorithm is based on having at least 15 pulses across the beam to provide a minimum of ±3% accuracy.

Another type of pulsed laser, operating in the kHz pulse rate regime is the Q-Switched laser. These lasers use the pulsing to increase, rather than decrease, their effective power. By concentrating the laser power into a short pulse, the peak power of each pulse increases while maintaining a low average power. In order to measure these lasers the same mathematical relationship of pulse rate to beam diameter applies, but there is an additional complication; the peak power of the pulses deliver sufficient energy to exceed the damage thresholds of the NanoScan even though the average power remains within the operating space. CW beams are measured as power (P) in Watts; pulsed beams as energy (E) in Joules. Therefore it is necessary to understand the beam's energy (Epulse) to determine whether the NanoScan can directly measure the unattenuated beam without incurring damage:

Therefore a beam with an average power of 300 Watts with a pulse frequency of 8kHz will have energy as follows:

The power density per pulse is also a function of the pulse duration τ. This is also important in understanding the potential damage to the profiler. Taking the above example, if the pulse duration is 1µs, then:

When the pulse duration of the laser gets very short, such as with pico- and femtosecond lasers, the peak power of the pulses can become very large. This creates some added complications when determining the type of scanhead that can safely measure these beams. In addition to the average power of the beam, which is used to determine the proper operating space of a given scan head, it is important to know the energy density of the pulses. The energy density must be below the damage threshold for the aperture material, and the average power must fall within the operating space of the scan head for it to be possible to measure the beam without additional attenuation. To determine the energy density, first use the above formula for the pulse energy:

Most pico- and femtosecond lasers have both a high repetition rate and a fairly low average power. They use the short pulse duration to amplify the effective power of the laser beam. A typical laser that one might encounter would have an average power of 1.0 watt and a repetition rate of 80kHz. For this laser the Epulse would be:

Using this value calculate the energy density for a given beam diameter by the following formula. Note that the energy density is presented as J/cm2; therefore the beam area needs to be converted to cm in the formula. Unless the beam is wildly different from round, it is easiest to consider that the area will be that of a circle:

Thus, for a 100µm beam at the 12.5µJ:

Once the energy density is calculated, it can be compared to the damage threshold for the aperture type and the wavelength range for the aperture material. The standard blackened slit material can only handle 10mJ/cm2 before the blackening starts to ablate. For this reason, scan heads intended for use with these pico- and femtosecond lasers should have the reflective slits, regardless of the detector type or the average power of the lasers. The wavelength of the laser also influences the energy density that the aperture material can withstand. For the standard slits the maximum energy density is 600mJ/cm2 for the range of 190nm to 400nm; for 400nm and above the value is 1.0J/cm2. For the high power copper slits the values are 2.5J/cm2 from 700nm to 3µm wavelength and 5J/cm2 above 3µm. Copper slits are not recommended for use below 700nm, however in some experiments we have seen better performance in the UV (@355nm) from copper slits. This may be attributable to the better heat dissipation of the copper material or the fact that the copper aperture material is thicker than the nickel alloy. The chart below can be used in lieu of the calculation to compare the energy per pulse at a given beam diameter with the appropriate threshold line for the aperture material and wavelength of use. For the above case the 12.5µJ energy at 100µm would be below the 600mJ damage line, but would certainly be well above the damage level for blackened apertures.

These estimates of damage threshold are primarily based on the relative reflectivity of the slit material. There are many other factors that may influence interaction of the laser beam and the aperture. At some level of power and pulse duration this interaction may become non-linear. In addition surface finish, roughness, contamination, tarnish or oxidation can also affect the reflectivity of the materials. For this reason these damage threshold values can only serve as a guideline, not an absolute guarantee. Use caution when measuring any new or unfamiliar laser system.

The following table gives a list of calculated minimum beam diameters at a given pulse frequency for each of the drum sizes and for a desired number of pulses per profile. The more pulses per profile the more accurate the measurement is likely to be. The formula is fairly simple. Due to the 45° angle of the slits to the direction of rotation, the actual speed of the slits is the drum speed divided by the square root of two.

The NanoScan pulsed operation can operate at any rotation rate, however it is recommended that the scan rate be 1.25 or 2.5Hz unless the laser repetition rate is above 50kHz. The larger drum used in the large aperture and High Power versions of the NanoScan cause the slits to move faster at any given rotation rate due to the larger circumference. For this reason the minimum beam sizes are larger for the large drum. The peak connect algorithm finds the highest peak pulse, then using the frequency value entered by the operator it finds the other peaks and connects them to generate a smooth beam profile. It is important that the exact pulse frequency be entered into pulse acquisition parameters. NanoScan provides this capability with all scan heads and detectors. Beams with average powers that were too low to be measured with the pyroelectric detector can now be profiled using silicon or germanium scanheads.

At high laser repetition rates (>100kHz) it may be better to operate the NanoScan in CW mode and let the auto filter smooth the beam. When this is preferable is dependent on the individual laser's pulse performance. If inconsistent results are seen with a high rep rate laser, it would be advisable to try the measurement both ways.

113995 does not work for the large aperture germanium detector NanoScan, which has a 12mm aperture

The ability to measure focused FIR beams has some real practical applications in industry. The sealed CO2 laser with powers levels from 10 to 250W are often incorporated into light industrial welding and cutting machines and very commonly into marking machines. With the growth of the marking industry, manufacturers of marking machines are finding that the higher demand is putting pressure on their operations to increase productivity and throughput. Where it once was acceptable to use crude methods such as burn papers, popsicle sticks and targets on the wall to align the lasers during assembly, they are finding that these are just not suitable for high throughput manufacturing. The use of a NanoScan to provide both a beam size and beam pointing measurement at the point of marking within seconds and speeds the process manifold. Rather than wasting precious time making inferior marks on test material, the optics can be adjusted on the fly using the NanoScan. The result is a perfect marking spot first time, every time.

Fiber lasers are known for their high beam quality and are finding their way into marking machines. The NanoScan is ideally suited to measuring and aligning these lasers as well.

Another area that needs the precision alignment and measurement capabilities of the NanoScan is the manufacture of precision devices, particularly in regulated industries where traceability of the laser performance is an absolute necessity. The NanoScan can guarantee that the laser at each workstation provides the same welding performance to the devices.

Rapid prototyping or "3-D printing" applications use lasers to sinter layers of material into three dimensional objects. These instruments also require that the beams be precise in both their beam size and pointing characteristics. Measuring the laser performance at the work plane ensures that the laser and the associated scanning optics are operating properly and are aligned for accurate reproduction of the parts.

Whether the laser is a CO2 at 10.6µm or an Nd:YAG at 1064nm, the pyroelectric detector equipped NanoScan can provide accurate, NIST-traceable beam measurement to ensure that the laser is performing up to expectation and specifications. No more arguments, no more uncertainty.

• NanoScan
• High-Power NanoScan
• Determining Damage Thresholds for Laser Measurements with a Slit-Based Profiler
• Determining Damage Thresholds for Laser Measurements with a Slit-Based Profiler