## 2 Instruments and Methods
### Spatially Resolved Acoustic Spectroscopy
If the interrogation area is larger than the crystallographic grain size, then the predominant velocity in the area is measured--the material texture is imaged instead [9; 10]. A SRAS system used to demonstrate the technique was first reported on in 2014 [18]. This system uses an IR (1064 nm) Q-switched pulsed laser (AOT-YAG-10Q, Advanced Optical Technology Ltd., Basildon, UK) to generate surface acoustic waves. In order to create the amplitude pattern that is incident on a sample, a mask made of quartz with a chrome coated grating is imaged on to the surface using the IR laser. The pulse duration of the IR laser is important as it defines the available bandwidth of the frequency within the generated SAW packet--in this setup, the pulse width of the laser is less than 2 ns. As SRAS is intended to be a non-destructive evaluation (NDE) technique, the system operates in the laser ultrasonic thermoelastic regime where waves are created in a solid sample by the sudden expansion due to localised heating--this is in contrast to the laser ultrasonic ablation regime where sufficient energy is applied such that the recoil the material experiences from the process of evaporation generates the ultrasound wave. The energy per pulse of the IR laser used in this setup is between 50-150 \(\upmu\)J at a repetition rate of 2 kHz. In the standard configuration, this system images eight lines with a period \(\lambda_{g}=24\)\(\upmu\)m. A 200 mW green (532 nm) continuous wave (CW) laser (Laser Quantum Torus), focused down to \(\diameter8\)\(\upmu\)m, is used for detection. As a SAW approaches the point where the detection laser is incident, the perturbation of the surface slightly changes the angle of the returned beam. In this setup, the reflected beam is measured using a knife edge detector (KED)--a detector based on measuring optical beam deflection (ODB) [19]. The KED contains a split photodiode, with the optical system designed such that the beam return is incident exactly between the split photodiode. The photocurrent generated is proportional to light intensity incident on each photodiode, and as the sample surface is perturbed (due to the SAW), a wave packet signal is generated off both photodiodes, opposite in phase. A differential amplifier circuit is then used to amplify the signal and remove common mode. A complete image of the SAW velocities are made by raster scanning the inspection sample using two high precision translation stages (M-531.DD, Physik Instrumente (PI) GmbH & Co. KG, Karlsruhe, Germany), which provides up to 300 mm of travel in two dimensions. The robustness of this system allows for single shot measurements to be made (i.e., no signal averaging required). A single direction SAW velocity is rapidly determined by performing a fast Fourier transform on the measured waveform, finding the maximum frequency response, and calculating the velocity using the associated frequency--the value of the frequency response is proportional to the measured signal power. Given the repetition rate of the IR laser, the scanning scheme implemented, and measurement overheads, the typical scan speeds are between 1000-1500 pts/s. To obtain the SAW phase velocities, the mask is rotated in order to propagate the generated surface waves in different directions--the detection laser also follows with the rotating pattern. Using the described generation and detection lasers, this setup has been used to image the microstructure of titanium, nickel, steel and aluminium based alloys [13; 14; 15; 20], as well as non-metallic materials such as polycrystalline silicon samples (once a metallic coating is applied) [16]. One of the main restrictions of this system (but not of the technique in general) is that the inspection sample requires a smooth, mirror-like surface, or it must be prepared through polishing. With a smooth non-transparent surface, the beam profile of the detection laser is mostly preserved when reflected, in this case a singular Gaussian beam.