Zhaoli Zeng

Note: High-performance HeNe laser feedback interferometer with birefringence feedback cavity scanned by piezoelectric transducer

We demonstrate a high-performance HeNe laser feedback interferometer (HLFI) based on a birefringence feedback cavity scanned by a piezoelectric transducer (PZT). The PZT scanning technique leads to a significant improvement in the HLFIs anti-jamming capability. The null drift under general laboratory conditions is approximately ±32 nm, corresponding to ±2 pulses. The phase difference between the two measurement signals does not vary when the feedback cavity length changes. This ensures a large measurement range for the HLFI. The HLFI presented here features a large measurement range (>1 m), nanometer-scale resolution (15.82 nm), a compact configuration, and low cost.

High-accuracy self-mixing interferometer based on single high-order orthogonally polarized feedback effects

A simple and high-accuracy self-mixing interferometer based on single high-order orthogonally polarized feedback effects is presented. The single high-order feedback effect is realized when dual-frequency laser reflects numerous times in a Fabry-Perot cavity and then goes back to the laser resonator along the same route. In this case, two orthogonally polarized feedback fringes with nanoscale resolution are obtained. This self-mixing interferometer has the advantages of higher sensitivity to weak signal than that of conventional interferometer. In addition, two orthogonally polarized fringes are useful for discriminating the moving direction of measured object. The experiment of measuring 2.5nm step is conducted, which shows a great potential in nanometrology.

High density fringes and phase behavior in birefringence dual frequency laser with multiple feedback

The high density intensity fringes and phase behavior in birefringent dual frequency laser with multiple feedback are studied for the first time. It was discovered that the fringes of output intensity are made of bipolar pulses with symmetric external cavity feedback and the fringe density is as high as compared to the conventional feedback. The high density cosine-like fringes are obtained with asymmetric external cavity feedback by adjusting the tilt angle of the feedback mirror and the fringe density is about 22 times higher compared to the conventional feedback. Moreover, there is a phase difference between the two cosine-like fringes and the phase difference is varied with the change of the external cavity length. The experimental results and a theoretical analysis are presented in this work. These results offer a large increase in the resolution for the optical feedback interferometer with the birefringence dual frequency laser.

Laser feedback interferometry based on high density cosine-like intensity fringes with phase quasi-quadrature.

A novel laser feedback interferometry based on high-order feedback is presented and realized for the first time. The interferometer uses a birefringence dual frequency laser and a tilted feedback mirror with high amplitude reflectivity to generate high density cosine-like optical fringes. These optical fringes have nanoscale resolution. Particularly, phase quasi-quadrature between the dual frequency fringes is obtained because of the phase shift caused by the changes of external optical path length. This phase characteristic can be used to distinguish the direction of movement easily. Under typical room conditions, the systems resolution is 0.51nm in 850μm range, and its 2 min displacement accuracy is 5nm.

The frequency stabilization method of laser feedback interferometer based on external cavity modulation.

Laser frequency stabilization is one of the foremost factors that influence the accuracy of measurement using a laser feedback interferometer, especially when the system is in the presence of strong optical feedback. In this paper, we use external cavity modulation to demonstrate a simple and effective method of frequency stabilization for laser feedback interferometer with strong optical feedback. The external cavity modulation is realized by oscillating a feedback mirror driven by a piezoelectric ceramic, the stable feedback optical field is obtained that has strong performance of anti-disturbance. Meanwhile, we stabilize the laser frequency by the thermal frequency-stabilizing. These lead to the realization of stable laser feedback interferometer. The relative uncertainty of the frequency stabilization is less than 1 part in 10(7).

Controlling the duty cycle of the eigenstates in laser with multiple optical feedback

The polarization dynamics of a quasi-isotropic single-mode laser subjected to multiple optical feedback is presented. The variable duty cycle of two eigenstates is observed in high-frequency optical fringes. The high-frequency optical fringes are induced by the multiple reflections in the asymmetry feedback cavity. The duty cycle of two eigenstates can be controlled easily by adjusting the position of polarization flipping due to the residual stress of laser mirror. Particularly, when the frequency difference results from residual stress is reduced to 1.5MHz, the position of polarization flipping moves to the edge of each fringe which can be used to measure small displacement with direction sensitivity and high resolution.

Polarization characteristics of He–Ne laser with different directions of polarized feedback

A polarizer is placed in an external feedback cavity to form polarized optical feedback. The effect of the different directions of polarized optical feedback on laser polarization characteristics (LPCs) is investigated experimentally and theoretically. The angle between the optical axis of the polarizer and the laser polarization is changed from 0° to 90°. It is found that LPCs vary greatly under different directions of polarized optical feedback. The angle range can be divided into five zones (two flipping zones, ⊥-polarization zone, bistable zone, and ∥-polarization zone) according to the different LPCs. When the angle is in the range of the ⊥-polarization zone (∥-polarization zone), the laser outputs ⊥-polarization (∥-polarization). Thus, one can choose either ⊥-polarization or ∥-polarization by properly aligning the axis of the polarizer.

Observation of transmission enhancement phenomena in a laser resonant cavity

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.