# Frequency-Shifted Optical Feedback Measurement Technologies Using a Solid-State Microchip Laser
## 4 Applications of the Laser Confocal Feedback Tomography
### Laser Confocal Feedback Imaging Combined with Other Technologies
#### 4.4.2 Ultrasound Modulated Laser Confocal Feedback Imaging  
When imaging objects inside turbid media, the number of ballistic photons decreases rapidly, compared to the scattered photons, as the penetration depth increases. As a result, the SNR is greatly reduced. The quality of the laser confocal feedback imaging is also affected by the scattered photons [39], and the generated speckles reduce the image contrast as well as the SNR. To solve the problem, [PERSON] [97] applied the ultrasound-modulated technology in the laser confocal feedback imaging. The system is shown in Figure 26. Except for the optical path focused into the sample, the ultrasound transducer is utilized, and the ultrasonic wave is focused into the same spot as well. As a result, the photons in the focal region are modulated with the shift frequency of the ultrasonic driving frequency [98,99]. Thus, the interesting photons are distinguished from the other scattered photons as the noise photons fall outside the focal region in the frequency domain. This is promising for reaching a larger penetration depth as well as a better SNR due to the reduction of noise photons.  
Figure 25: Experiment results. (**a**) Image obtained by directly scanning. (**b**) Image after filtering with a DL of 48.5 mm. The three insets on the right are the magnified images that correspond to the rectangular regions labeled in the image. (**c**) Image after filtering with a DL of 53.5 mm. (**d**) Image after filtering with a DL of 58.5 mm. (Figure reproduced from Ref. [96]).  
When the measuring light returns to the laser cavity, the optical power modulation can be deduced as:  
\[\frac{\Delta I(F_{a}+2\Omega)}{I_{\mathrm{s}}}=M_{1}I(u)\kappa G(F_{a}+2\Omega) \cdot\cos((F_{a}+2\Omega)t-\phi+\phi_{s}) \tag{21}\]  
where \(\Delta I\) denotes the intensity modulation of the measuring light, \(I_{\mathrm{s}}\) is the output power of the solitary laser, \(M_{1}\) is the ultrasound one-sided modulation depth, \(I(u)\) is the light intensity of the confocal system, \(\kappa\) is the coefficient of the feedback light strength, \(G\) is the frequency dependent amplification factor, \(\phi_{s}\) is a fixed phase, and \(\phi\) is the phase related to the external cavity length.  
The simulations and experiment are conducted in the paper. The contrasting experimental results with the traditional laser confocal feedback imaging are shown in Figure 27, proving that it is an effective method to realize a better SNR with the increasing of the scattering coefficient and focused depth. Compared with other optical methods, it is possible to reach both a larger imaging depth and a better SNR, especially in the turbid media.  
Figure 27: Results of experimental imaging: (**a**) laser confocal feedback imaging, (**b**) ultrasound modulated laser confocal feedback imaging. (Figure reproduced from Ref. [97]).  
Figure 26: Schematic diagram of the ultrasound tagged laser confocal feedback imaging system. ML, microchip Nd: YVO\({}_{4}\) laser; BS, beam splitter; PD, photodiode; L, lens; AOM\({}_{1}\), AOM\({}_{2}\), acousto-optic modulators; BE, beam expender; R, reflector; Obj, objective; Sa, sample (phantom); UT, ultrasound transducer; Z, the axis of the incident light. (Figure reproduced from Ref. [97]).