The strong optical feedback has the advantage of generating high resolution fringes. However, these feedback fringes usually seem like the noise signal when the feedback level is high. This defect severely limits its practical application. In this paper, the generation mechanism of noise fringes with strong optical feedback is studied by using spectral analysis method. The spectral analysis results show that, in most cases, the noise-like fringes are observed owing to the strong multiple high-order feedback. However, at certain feedback cavity condition, there may be only one high-order feedback beam goes back to the laser cavity, the noise-like fringes can change to the cosine-like fringes. And the resolution of this fringe is dozens times than that of the weak optical feedback. This research provides a method to obtain high resolution cosine-like fringes rather than noise signal in the strong optical feedback, which makes it possible to be used in nanoscale displacement measurements.
The intensity and polarization dynamics of a laser subjected to anisotropic high-order optical feedback have been investigated. The feedback system is realized by a high-reflectivity feedback mirror and a wave plate in the feedback cavity. The high-resolution optical fringes with nanometer order are obtained owing to the high-reflectivity feedback mirror, and the modulation depth of these optical fringes is relatively uniform, which is different from that of instable fluctuation or intensity noise that has been previously observed. In particular, the polarization flipping is found in each fringe, and the flipping position can be easily changed by rotating the wave plate in the feedback cavity. Furthermore, when the flipping position moves to the edge of each fringe, this optical fringe becomes promising for use in precision measurement. The theoretical analysis based on the compound cavity model and the Floch rotation flipping mechanism is presented, and it agrees well with the experimental results.
We present the experimental observation of a phenomenon in which the reflection loss, induced by an uncoated glass sample placed in a laser cavity, significantly reduces at a series of incident angles. The light amplification condition for a laser to work can be satisfied by means of this phenomenon, though the gain is less than the loss when the sample is placed in the normal incidence. The angle ranges for the laser can keep working are intermittent, and both of the lasing range and no-lasing range become narrow with the incident angle increasing. Six kinds of optical glass samples and one birefringent sample have been tested, and three types of lasers are used to confirm this phenomenon. This phenomenon may make the anti-reflection film be not necessary for a transparent sample in some techniques or instruments based on the characteristics of laser resonant cavity. Principle and properties of this phenomenon are analyzed, and the theoretical analyses are coincident to the experimental observations. Three conditions for this phenomenon to occur, as well as the potential applications, are given finally.