Spatial Light Modulator (SLM)

A Hamamatsu Photonics phase only spatial light modulator (SLM) was used to divide a single input beam into multiple beams for parallel processing. The SLM display (figure 35) is attached to a controller unit and a computer; the LCD acts as a secondary monitor screen. Using the software provide by Hamamatsu, different CGH’s can be created and then moved to the second screen (LCD). The CGH’s used during processing are shown in the relevant chapter.

Figure 35: SLM in optical path. Beam is directed onto the LCD were user defined computer generated hologram (CGH) are displayed. The CGH is determined by the user and can be used to generated multiple beams or as a method for pulse shaping.

CGHs created on the computer are converted and displayed on the liquid crystal display by applying a discrete voltage to individual pixels. As a result an electric field associated to each pixel is established causing a change in the refractive index of the liquid crystal. The change in refractive index (∆n) is used to display the hologram and enables modulation of the incoming light. Phase modulation can be described using the following

Where is the optical path difference, is the phase change, k is the wave number, λ is the wavelength and d is the thickness of the liquid crystal.

The SLM used in this study displays 8 bit CGH, where voltage variation on the CCD pixels has 256 levels (28); therefore the phase change of the reflected light will vary as a function of the grey level. Figure 36 shows a near linear increase in phase modulation with increasing grey level.

Figure 36: Results measured from a Holoeye SLM showing a near linear increase of the pahse modulation with varying grey level.

Figure 37: Nutfield scanning head positioned on the HighQ system. Two galvometer mirrors directed the beam through a 100mm flat field lens.

3.7 Nutfield scanning head

To operate the laser beam for both microfabrication and restoration a Nutfield scanning head (model: XLR8) was used. The scanning module consisted of two galvanometers, two optics and integrated driver electronics. Each galvanometer controlled movement in one of either X or Y, whilst flat field lenses were used to focus the beam on the target area. This combination allows the system to be configured to deflect laser pulses for the majority of applications. The maximum working field of the scanning head is 70 x 70 mm, with a resolution of <4 μm. For most of the work undertaken in this study this scanning field was sufficient, in cases where this was too small edge effects were avoided by integrating scanning with movement of the Aerotech stage.

3.8 Aerotech motion and positioning control system

Throughout laser processing accurate control of both the position and location of the sample relative to the beam is very important. Deviation from the required parameters can

Figure 38: Aerotech stage (left) and driver unit (right). This stage was attached to the HighQ laser and provided movement in 5-axis XYZUA.

The Aerotech stage can provide high accuracy movement through three or five degrees of freedom (figure 38), depending on the laser system, with a resolution of 0.5μm in all directions.

This CNC stage was controlled by the Npaq driver and Nview GUI. Commands were either entered directly into the software using the Nview programming interface or loaded from any other CNC language. Movement of the stage can be defined in two ways, as either absolute co-ordinates (with respect to the table origin) or as relative co-ordinates (with respect to the last position); this allows for both independent and directed axis movement. In addition it was possible to define values for the traverse speed and vector speed (directed movement), acceleration and deceleration and shutter control.

3.9 Scanner application software

During this study two different software programs were adopted to design the microstructures presented later in this thesis: SamLight and Waverunner. SamLight was used for operating the HighQ whilst Waverunner was used on the Fianium system; the user interface for both is very similar. In the grid area in the centre of the screen various patterns can be drawn or loaded from other files. The laser operating parameters such as traverse speed, number of repeat scans, hatch spacing and hatch type can be specified in this software.

3.10 Tektronix AFG 3021B function generator

Figure 39: Tektronic function generator; using this system it was possible to gate the laser to emit a specific number of pulses. This enables the user to produce frequencies up to the maximum produced by the system.

As mentioned, it was possible to extract a specific number of pulses from the HighQ laser system, this cannot be done with the standard software (Highq.exe); to achieve this a Tektronic AFG function generator (figure 39) was used. The AFG can produce different functions including: continuous, square, burst and sine waves. Within these functions it was possible to alter different parameters including frequency, width and delay. By selecting suitable functions and parameters to trigger TTL signals, various pulse frequencies were achieved.

3.11 Analysis Equipment

3.12 WYKO NT1100 White Light Interferometric Microscope

In addition to the Nikon digital microscope a WYKO NT1100 interferometric microscope was employed to collect further information about the surface pre and post processing. The WYKO NT1100 is a non-contact optical profiler which provides accurate images of the surface topography in three planes (XYZ). The resolution in Z was accurate to a few nanometres; this allows for precise measurements to be made. In the XY planes the

Figure 40: White light interferometry profiling system. A light source was used to illuminate the surface of a target object; interference fringes at the focal plane are used to produce an image. Measurements were initiated and analysed using the Vision software provided.

A white light source is passed through a beam splitter separating the beam into two individual beams of equal intensities. Half the beam is delivered to a reference mirror and the other half to the test surface; the reflected light from both surfaces is then recombined generating an interference pattern. Due to the low coherence of white light, interference fringes are observable only over a short range for each focal position. During a scan several measurements were taken over a user determined range and recorded using the Vision software.

During post processing the software recalls saved data and selects the vertical position corresponding to the peak fringe signal for each point on the surface; the data obtained is decoded and combined creating a complete 3D picture of the surface morphology.

Interferometric measurements were taken by placing a sample on an adjustable XY stage; this ensured the area of interest was illuminated.

The Z position of the stage was altered to ensure that the sample was located at the focal point where interference fringes were observed; the number of observable fringes can be adjusted. Before measurement can take place the objective and field of view (FOV) needed to be checked to ensure that the values shown in the software match the interferometer; this made certain that accurate readings were recorded. For some samples, the amounted of reflected light was increased to ensure a strong enough signal to record an image; in these instances samples were placed on a polished stainless steel surface.

Once at focus and the software parameters had been checked a scan was performed on the sample; following this procedure ensured that measurements were accurate. Using the Vision software data for observing the depth of ablation, surface roughness and topography were obtained.

The system is calibrated at periodic intervals by measuring against a standard sample provided with the microscope and comparing results.