# Frequency-Shifted Optical Feedback Measurement Technologies Using a Solid-State Microchip Laser
## 1 Introduction  
Laser feedback, also known as laser self-mixing interference, was first applied as a displacement sensor by [PERSON] in 1963 [1]. It is a physical phenomenon where part of the output laser is reflected or scattered by the external object returning back into the laser resonator to modulate the laser output power, phase, polarization states, and so on [2; 3; 4; 5; 6]. Unlike the traditional laser interferometry, laser feedback interference occurs inside the laser resonator, thus making the system concise and auto aligned. In the 1970s, the laser feedback effect of the laser diode (LD) became one of the important research fields due to the development of the optical communication technology. The theoretical system of the LD optical feedback was established by [PERSON], [PERSON], [PERSON], among others [7; 8; 9; 10]. However, the sensitivity of the LD optical feedback is still not high enough to realize the non-cooperative measurement as the black or high transmission targets. In 1979, [PERSON] reported the external optical feedback of LiNdP\({}_{4}\)O\({}_{12}\) lasers produced using a rotating glass plate [11]. This kind of laser, known as the microchip laser, belongs to the class-B laser, of which the population decays slowly compared with the field [12]. In this case, the feedback signal can be amplified by a gain factor of the ratio \(\gamma_{c}/\gamma_{1}\), where \(\gamma_{c}\) is the damping rate of the laser cavity and \(\gamma_{1}\) is the damping rate of the population inversion. According to the analysis of classic rate equation, the ratio \(\gamma_{c}/\gamma_{1}\) of a microchip laser can be high as \(10^{6}\). The ratio is onlyfor an LD laser, and much lower in a gas laser. Therefore, the microchip laser has a high sensitivity compared with other kinds of lasers. It has been applied in various fields, such as displacement sensing [13; 14], velocimetry [15], vibrometry [16; 17], particle detecting [18], angle measurement [19], and laser parameters measurement [20; 21]. Imaging objects inside turbid media [22; 23; 24; 25], such as the biological tissue and scattering liquid, is a challenging problem in many fields. Due to the non-contact, excellent contrast, high-resolution, nonionizing, and noninvasive, optical imaging is currently emerging as a promising method in medical imaging [26] (pp. 1-8) through the use of confocal microscopy [27; 28; 29], diffuse optical tomography [30; 31], fluorescence spectroscopy [32; 33], and optical coherence tomography [34; 35; 36]. For diffuse optical tomography, the penetration depth can be several centimeters into biological tissue, but recovering information from scattered photons is a challenge, and the spatial resolution is on the order of 20% of the imaging depth [26] (p. 249). Most optical imaging methods are based on ballistic photons, which will be rapidly decreased when the penetration depth increases in turbid media; thus, these methods are usually confined to imaging of a few millimeters in biological samples. More recently, the laser optical feedback tomography was demonstrated by [PERSON] [37], which realizes the surface imaging of a French coin immersed in 1 cm of milk. Combining the technology of confocal tomography and a laser feedback effect, [PERSON] et al. proposed the laser confocal feedback tomography [38; 39]. Due to the ultrahigh sensitivity of a solid-state microchip laser to external frequency-shifted feedback, it shows promise towards reaching a greater depth than other methods. This paper provides an overall review of the research on frequency-shifted optical feedback measurements using a solid-state microchip laser. The remainder of the review is structured as follows. Section 2 reveals the basic principles and experimental setups of the laser feedback interferometer and laser confocal feedback tomography.