Diagnostic technique for verifying the phase stability of SLMs under high-average-power laser

2.3.1 Aim of the experiment

The aim of the research presented in this thesis is to control the polarization and phase of laser beams to improve industrial micro-processes. In order to optimize efficiency, industrial processes often use high-average-power beams. These high-average powers can be incompatible with some of the delicate optics used in a research laboratory. Here, I will demonstrate that the optics I use for polarization and phase control are sufficiently robust to be used at high-average-power in an industrial environment. One of the key optics used in the experimental setups here is a phase-only liquid-crystal SLM from Hamamatsu. This type of SLM has often been used with low-average-power beams in a research environment, but its suitability for high-average-power (and hence industrial) use needs to be confirmed. An SLM works by modulating the phase of a laser beam. In this section I will verify if exposure to a high-average-power beam affects the phase modulation induced by an SLM. This is done by programming an SLM to induce a phase grating which splits the beam into a zero- and a first-order component. The resulting diffraction efficiency, which is defined as the power-ratio between first-order beam and full reflected beam (i.e. first-order plus zero-order), is used to evaluate the stability of the phase modulation. A fluctuation of diffraction efficiency as a result of a long exposure to a high-average- power beam would imply that the phase modulation is affected by the exposure to the beam. Here, the diffraction efficiency is measured while the SLM used in this experiment is exposed to a laser beam with 8W of average power for 4.5 hours.

2.3.2 Experimental setup

Figure 2.7 shows the optical setup used to check the phase stability of the SLM. A 532nm, 150ns pulse laser beam set at 8W of average power output is incident on a reflective phase-only SLM from Hamamatsu (model X10468-04). The SLM induces a diffractive phase grating which splits the reflected beam into a zero-order beam and a first-order beam. After the SLM, an iris is used to discriminate the first-order beam against the zero-order beam. When the iris aperture diameter is reduced, only the first- order beam is transmitted, while the zero-order beam is blocked. When the iris aperture diameter is at its fully open position, both the zero- and first-order beams are transmitted (i.e. the full beam power reflected from the SLM). A power meter is placed after the iris to measure either the first-order or the full reflected beam power, depending on the chosen aperture of the iris. A beam splitter is also inserted in the beam path before the SLM to pick-off a fraction of the beam. Another power meter measures the power from this beam pick-off. In this way, the stability of the output from the laser source can be verified during the experiment. The readings from the three power measurements are plotted against the duration of exposure in Figure 2.8-a. A slight increase in all three power readings is noted in the first 30 minutes. This is due to a 5% power increase in the output from the laser source, demonstrated by the power measurements from the pick-off (not visible in Figure 2.8 due to the chosen scale). It is therefore irrelevant to this experiment. A value for the diffraction efficiency is derived by dividing the

Figure 2.7: Schematic showing the experimental setup used to check the phase behavior of the SLM under high-average-power laser exposure. The laser source is a 532nm, 12kHz repetition frequency, 150ns pulse laser. For these tests, the average output power is set at 8W. The iris aperture diameter is reduced to measure the first-order beam power, or fully open to measure the full reflected beam power.

Figure 2.8: (a) Power meter readings during laser exposure. (b) Diffraction efficiency derived from the power measurements.

A diffractive phase grating splits the incident beam into a zero- and a first-order beam component Iris First-order SLM Beam splitter Power meter Zero-order Power meter BMI laser (532nm)

a)

b)

power value for the first-order beam by that of the full reflected beam. The diffraction efficiency is plotted in Figure 2.8-b.

2.3.3 Results and discussion

It can be seen from Figure 2.8-b that the diffraction efficiency is relatively stable (within +/-3.5%) over the duration of the exposure (4.5 hours). If the phase response of the SLM was affected by a continuous high-average-power laser beam exposure at the 8W level, the diffraction efficiency would be expected to change significantly. As the diffraction efficiency remains reasonably constant for the duration of the experiment, we can conclude that the phase response of the SLM is unaffected under these experimental conditions.

Here, I have demonstrated that the SLMs used in this research are compatible with an average beam power of 8W, which is representative of the average-power used in some industrial applications. In the rest of this thesis, the experiments that use SLMs will involve lower average beam powers. However the experimental results above give confidence that the SLM-based optical setups introduced in this thesis are potentially transferable to industrial applications.

In the rest of this thesis, several experiments use SLMs exposed to high-peak-power ultrashort-pulse laser beams (with a low average-power P<3W). The phase modulation induced by the SLMs has remained constant throughout and no optical damage has been noticed on the active liquid-crystal layers of the SLMs. As for high-average-power beams, there is no evidence to suggest that high-peak- power beams affect the phase response of the SLMs in these experiments. It is noted that Hamamatsu has tested its SLMs to peak power densities above 10GW/cm² without producing any damage to the liquid-crystal layers.