## 2 Instruments and Methods
### Speckle Knife Edge Detector  
A previously reported detector developed by the authors, known as the speckle knife edge detector (SKED) [28; 29], has been implemented as an all-electronic chip based device. As a concept, the SKED works as an expanded knife edge detector--in place of the two adjacent photodiodes there is an array of photodiodes. In the scenario where a speckle field is incident on the array, each speckle can be considered as an individual detection beam. Assuming that the size of each speckle is larger than two photodiodes, a knife edge 'photodiode pair' can be established, where each side of the 'pair' can contain the signal measured by multiple photodiodes. The signal obtained by measuring the deflection on all the incident speckles can be combined. An example speckle pattern split is shown in Figure 2b. For the SKED concept to function, the output channel that each individual photodiode is connected to must be configurable. This process can be automated by observing the intensity gradient across a selection of pixels (usually a row or column) and grouping sets of pixels to the two different outputs  
Figure 2: An illustrative laser ultrasound detection outline on a (**a**) smooth, (**b**) rough surface. On a smooth surface (**a**), the small deflection of a singular return beam can be measured using a knife edge detector, however, on a rough surface (**b**), the change in intensity seen on either photodiode due to the deflection of the speckle field is not detectable. The speckle knife edge detector (SKED) ‘splits’ the incident speckles into left/right channels across its detector array, allowing it to detect the small deflections in the speckle field. based on when an intensity zero crossing occurs, i.e., when there is an apparent maximum or minimum in intensity, effectively splitting a speckle. As the SKED concept is classified as an OBD detector, it can still be considered robust in the presence of environmental vibration. A number of custom SKED silicon chips had been developed based on this concept. The latest iteration was fabricated using a 0.35 \(\upmu\)m CMOS process--a \(32\times 32\) pixel array with 62 \(\upmu\)m pitch and 27% photosensitive area. Simulations of the chip show that the time required to switch between a configuration state and signal output is less than 0.5 \(\upmu\)s (equivalent to speckle adaption time), meaning theoretically 1 MSamp/s is possible. The simulated 3 dB bandwidth is 46 MHz when half the pixels are connected to the outputs (the bandwidth of the SKED increases as the number of pixels connected to the output decreases). To interface with the SKED chip, PCBs have been designed to output the detected ultrasound signal and to control the chip through USB via a PIC microcontroller--the most recent version is contained within a \(\diameter 31.5\) mm metal tube, which allows for a SKED device to be mounted inside an optical 35 mm cage plate holder (as shown in Figure 3a,b). To enable measuring on rough surfaces, a free-space benchtop SRAS setup was designed and built. A photograph of the setup is shown in Figure 4. Aside from the detection aspect, the other major consideration is the propagation of ultrasound on the rough surface. The attenuation of the wave is a complicated problem, as it is dependent on the surface wave frequency and the characteristics of the rough surface--the attenuation rate along a determined length is proportional to the fifth power of the frequency of the Rayleigh wave given that the wavelength of the Rayleigh wave is longer than the transverse correlation length of the surface roughness [30; 31; 32]. One of the key advantages of using SRAS is that the surface wave can be detected adjacent to the wave generation area--this is in contrast to other laser ultrasound techniques that measure time-of-flight and require a suitable distance between generation and detection for accuracy. The rough surface SRAS setup was designed so that the mask wavelength (\(\lambda_{g}\)), and therefore frequency (\(f_{s}\)), can be easily changed.