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Ultrasound-modulated optical imaging technology is a new type of biological tissue optical detection technology, and sensitive to the change of scattering coefficient and absorption coefficient of biological tissue. This technology is a non-ionizing and non-invasive pathological detection method, which has great potential application in early detection of cancer. However, ultrasound-modulated optical imaging technology is insufficient in signal-to-noise ratio (SNR) and imaging contrast. Frequency-shifted laser feedback technology with microchip laser is a new type of highly sensitive interference technology, whose gain coefficient for weak optical signal can reach 106. This technology can greatly improve the SNR of imaging. Combined with the high sensitive laser feedback technology, the ultrasound-modulated laser feedback technology is proposed. The SNR of this technology is better than that of the traditional ultrasound-modulated optical imaging technology. The increase in SNR can achieve greater depth of detection in biological tissue imaging, but there is no significant improvement in imaging contrast. In order to improve the contrast of biological tissue imaging and achieve high resolution imaging of thick biological tissue, we use ultrasound microbubble contrast agent in ultrasound-modulated laser feedback imaging technology. We establish a Monte Carlo photon transport model with microbubbles in order to study the mechanism of contrast enhancement in ultrasound-modulated laser feedback imaging with microbubbles. Finally, we establish an experimental system to verify the correctness of the simulation results. Experimental and simulation results show that in the transparent solution, the ultrasonic microbubble contrast agent can enhance the ultrasound-modulated laser feedback signal and generate harmonic modulation, which can improve the imaging contrast by detecting the enhancement of the fundamental and harmonic signals of the feedback. In the scattering medium, the ultrasonic microbubble contrast agent can significantly attenuate the ultrasound modulated laser feedback signal, and the imaging contrast can be improved by detecting the attenuation of the fundamental and harmonic signal.

Microchip laser can output two orthogonal frequency splitting modes due to its own internal residual stress. In this paper it is found that the frequency difference of microchip laser is modulated by laser feedback. The modulation is a sinusoid-like curve whose center is the original frequency difference and period is half-wavelength. The amplitude of the modulation curve is proportional to feedback level. But when the feedback is too strong, so that the polarization switching occurs and only one polarization exists sometimes. In a range of external cavity, the amplitude of the modulation curve is also proportional to original frequency difference. The theoretical analysis and simulation based on the composite laser cavity theory and self-consistent theory are in good agreement with the experimental results. The potential applications of this phenomenon in precision measurement are discussed.

Optical feedback characteristics of frequency modulated microchip Nd：YAG lasers are investigated. Without optical feedback，the output intensity changes by one fringe with the laser frequency changing by one longitudinal mode spacing. With optical feedback，the modulation frequency of output intensity is proportional to the ratio of external cavity length to internal cavity length. The theoretical analysis based on three-cavity-mirror equivalent model is in good agreement with experimental results. The potential applications of this phenomenon in precision measurement are also discussed.