High power lasers – just measure it

Author: 
Yoni Groisman

Innovative attenuation device under test at Laser Zentrum Hannover (LZH)

Laser power and the applied laser densities are constantly rising. These high-power lasers are more likely to be used in materials processing applications such as welding, cutting, cladding, or additive manufacturing. Many of those applications are demanding: The laser's focal spot and beam profile need to be kept within the specification to achieve the required part quality and establish a reproducible process.

How can laser parameters of beams in the range of multi kilowatts and with power densities reaching more than 10 MW/cm2 be measured with existing camera technology without damaging the camera or the optics?

Unprecedent attenuation
Laser power attenuation via beam splitting is a common practice to reduce the laser power and enable the measurement of a beam. For a long time it has not been possible to attenuate a beam with powers above the kilowatt range.

 

Enter the Ophir LBS-300HP-NIR beam splitter, which can be used with laser power densities of up to 15MW/cm2 at 5kW. High power lasers – just measure it
Fig. 1. LBS-300HP-NIR with SP932u Beam Profiler
The key advantage of this innovative device is the uniform attenuation of any beam shape (Gaussian, flat-top, doughnut, etc.) while fully preserving the polarization and overall profile of the incoming laser beam. Beam shape, focal spot, beam waist, and overall power of the NIR laser beam up to 5kW can thus be reliably measured.

 

As with every innovative technology, users need additional assurance that the promised results can really be achieved. The key points of interest with a device that attenuates the beam are:

  • Can the attenuator itself be damaged by the beam?
  • Will a temperature increase of the device influence the measurements?
  • Does the device deliver reliable results for CW and pulsed lasers?

In order to provide proven results by a third-party institute, the well-known German research institute Laser Zentrum Hannover (LZH) conducted a series of tests with high power industrial lasers (click here to get the full report).

 

Sophisticated technology
Uncoated optics provide insufficient attenuation and most coatings are vulnerable to both CW and pulsed laser induced damage. The LBS-300HP-NIR beam splitter solves this key struggle by combining specially surface treated wedges and adjustable ND filter slides to reduce laser power density from 15MW/cm2 down to 15 µW/cm2. The incoming beam is reflected through the front surfaces of a pair of orthogonally oriented wedges. Less than 0.0001% (1/106) of the beam is then reflected towards the beam profiler camera and less than 0.1% towards the optional power meter. The remaining 99.9% of the incident laser beam is transmitted

 

High power lasers – just measure it
Fig. 2. Example Setup to measure high power beams.
High power lasers – just measure it
Fig. 3. An example of 1.4 kW fiber laser core and ring as measured via LBS-300HP-NIR attenuator.

 

Extensive testing with CW lasers
The engineers at LZH performed several tests with the beam splitter at the institute by using both CW and pulsed lasers. The first irradiation tests of the Ophir LBS-300HP-NIR were done with a Trumpf TruDisk 16002 laser at 1030 nm in CW mode.

Thermal stability and reflection test
The intention of the first test with the CW laser was to measure the temperature increase of the housing when applying different power levels to the beam splitter and the respective reflection at the different power levels to find out if an increasing temperature influences the reflection of the beam splitter. The LBS-300HP-NIR was irradiated with 2, 3.5, and 5kW for 15 minutes. Thermal monitoring was performed via two (2) thermopile sensors on the beam splitter housing. Before and after each irradiation the first wedge was visually inspected. Irradiating the sample with 2kW, 3.5kW, and 5kW did not induce any visual laser damages

 

Power [kW] Visual inspection Temp. after 15 min [°C] Attenuation Ratio
2.0 No damage observed 39 1.1∙106.
3.5 No damage observed 47 1.1∙106.
5.0 No damage observed 59 1.1∙106.
Table 1. Evaluation of test 1 – moderate temperature increase with higher power levels and constant attenuation levels

 

High power lasers – just measure it
Fig. 4. 3500W irradiation – reflection and power vs time (left) and temperature vs time (right). Attenuation ratio is approximately 1.1∙106. (more than 1 to 1 million)

This test clearly demonstrated that even though the housing temperature went up to around 50°C when exposed to a 3500 Watt laser for 15 minutes, the attenuation remained the same 1.1∙106 (more than one to 1 million). Even with 5kW the LBS-300HP-NIR showed stable attenuation.

Power density and laser induced damage testing
Another sensitive parameter in high power applications is the power density of the laser beam. Power density is defined as the ratio of the laser power over the area of the beam cross section and is considered to be a key parameter that can cause laser induced damage. Especially with small beam diameters, the power density of a high-power laser beam can quickly rise to MW/cm² values that need to be carefully taken into consideration when measuring a beam.

In order to analyze the endurance of the LBS-300HP-NIR to high power densities, a high-power Gaussian laser beam was focused on its first wedge. The beam caustic is shown in fig. 3; the effective beam diameter on the first wedge was determined to be 147μm.

High power lasers – just measure it
Fig. 5. The laser beam used for the tests showed the above Gaussian profile

The LBS-300HP-NIR was then irradiated for 2 minutes with power densities of 15, 17, 20, 23, 27, and 30MW/cm². After each irradiation the wedges were visually inspected.

 

Power density [MW/cm²] Visual inspection Temp. increase [°C]
15 no damage 4.7
17 no damage 4.0
20 no damage 3.8
23 no damage 4.7
27 no damage 5.5
30 no damage 5.8
Table 2. Visual inspection results before and after each irradiation at corresponding power density level and the temperature increase of the housing after 2 min

 

Individual evaluations have been prepared for each power density level showing that even though a temperature increase was measured on the housing, the percentage of the reflection was stable.

High power lasers – just measure it
Fig. 6. 30MW/cm² irradiation – reflection vs time (left) and temperature increase vs time (right).

The test clearly demonstrates the functionality of the beam splitter when used with a CW laser. As a next step, the system was tested using a pulsed laser.

Beam attenuation of pulsed lasers
The purpose of pulsed tests was to determine the high-energy pulses endurance of LBS-300HP-NIR, in both damage threshold, attenuation, and thermal stability.

LZH used an InnoLas solid state laser operating at 1064 nm with pulse frequency of 10 Hz and 8ns pulse width to conduct further tests of the LBS-300HP-NIR. The profile of the beam was measured with an Ophir SP620U camera.

High power lasers – just measure it
Fig. 7. Beam profile with an effective beam diameter of 233μm.

Energy Density – Temperature – Visual Inspection – Reflection
LBS-300HP-NIR was first irradiated for 2 minutes with energy densities (at 1st wedge) of 10, 15, 20, and 25 J/cm²; afterwards the wedges visually inspected.

High power lasers – just measure it
Fig.8. 25 J/cm² pulsed laser – reflection 1st wedge vs time (left) and temperature increase vs time (right). (The reflected beam was measured after 1st wedge, indicating only half of entire attenuation)

As demonstrated in all tests 10-25 J/cm² with pulsed lasers, the LBS-300HP-NIR device delivers both attenuation and thermal stability, with no laser induced damage. Camera based beam profiling is thus applicable to use with high energy pulsed laser beams.

Conclusion
Developing new laser processes with high power CW lasers as well as with high energy pulsed lasers is a challenging task for the engineers in material processing. No matter if it is in welding, laser-based metal printing, or other demanding applications – beam profiling is a key asset when it comes to precisely determine the beam characteristics and to develop sustainable processes. In the extensive testing performed at the LZH, the innovative technology introduced with the Ophir LBS-300HP-NIR beam splitter proved to withstand high power densities as well as high energy without interfering with the beam or changing measurement results. In addition to the non-contact measurement of high-power laser beams, engineers now have another valuable tool at hand to analyze high power laser beams.

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