## 3 All-Optical Ultrasound Transducers and Imaging
### Development of Transducers and Imaging
#### 3.1.3 Performance Enhancement  
Suitable integration of LGUS transmitters and ODUS detectors into an AOUST is not only for convenient operation, but also for keeping the superiority of high-frequency and broadband operation. The ultrasound transmission and detection of the piezoelectric transducer is naturally well coordinated because the acoustic wave is generated and detected through the same piezoelectric material. Unlike the piezoelectric transducer, the ultrasound transmission and detection based on the optical method should be well organized deliberately. Otherwise, there could be some issues such as a long propagation distance and/or a large incident angle from the ultrasound generation to detection, leading to attenuation and distortion of measured acoustic signals, especially for high-frequency components. In addition, one may suffer from the problem of demanding and time-consuming alignment in scanning microscopy and the difficulty of arranging the set up in a reflection-mode operation for convenient applications. Several decent approaches of integration are demonstrated. (i) A broadband AOUST consists of a 2D Au nanostructure on a glass substrate, followed by a PDMS layer and a Au layer [28; 29]. The Au nanostructure is for LGUS transmission, while the Au-PDMS-Au structure is used as an FP etalon for ODUS sensing. Pulse-echo ultrasound with a broad bandwidth of 57 MHz is demonstrated. High-frequency ultrasound arrays are also proposed through directing both the ultrasound generation and detection laser beams into an array of focused spots on the device surface. Another similar work employs black PDMS for LGUS transmission [89]. (ii) An improved design using a polyimide-etalon thin film structure is demonstrated [95; 96]. As for the AOUST using 2D Au nanostructure [28; 29], the excitation laser intensity is partly absorbed by the Au nanostructure and partly absorbed by the FP etalon, which results in damage to the FP receiver and is not suitable for long-term use. As for the one using black PDMS [89], the configuration prohibits transmitter and receiver elements from being in the same location, which is undesired for imaging purposes. The polyimide-etalon AOUST overcomes the above obstacles by utilizing the unique characteristics of the polyimide that is highly absorptive in a narrow spectrum, ultraviolet  
Figure 7: **(A)** The fiber-optic all-optical ultrasound transducer (AOUST) consisting of an optical fiber with a CNTs–PDMS nanocomposite coating for LGUS transmission and an adjacent fiber with an Fabry-Perot (FP) etalon for optical detection of ultrasound (ODUS) sensing. Tubular metal housing is also shown. **(B)** The imaging system for the AOUST probe. CW: continuous wave; GPIB: general purpose interface bus; PCI: peripheral component interconnect. Reproduced with permission from [77], OSA Publishing, 2015.  
range, for LGUS transmission and highly transparent to wavelengths, near-infrared range, for FP sensing. Thus, high lateral resolutions of 70 and 114 um at depths of 2.4 and 5.8 mm, respectively, with an average axial resolution of 35 um are obtained. (iii) An intriguing excitation method is proposed for AOUSTs for resolution and efficiency enhancement [97]. Pulsed laser excitation is commonly used to produce ultrasound with broad bandwidth. Energy loss could happen in certain frequency bands due to acoustic attenuation within tissues, nonuniform acoustic power spectra in generation or nonuniform acoustic sensing spectra in detection, leading to reduced system efficiency and bandwidth. By using an optical modulation for ultrasound excitation, the bandwidth can be constrained to those frequencies determined by the process of all-optical ultrasound transduction. Through pulse-echo imaging experiment, a linear chirp excitation presents similar axial resolution and signal-to-noise ratios (SNRs) compared with conventional pulsed laser excitation.