Shulian Zhang

Birefringent tuning double frequency He-Ne laser

Experimental details are given of a novel, promising kind of double frequency laser-with power output of 0.8-1.4 mW, large frequency difference from 37 MHz to a longitudinal mode interval, and frequency stability of 10(-5), and its potential uses are discussed.

Method for the measurement of phase retardation of any wave plate with high precision

We present a measuring method for any wave plate retardation with fairly high precision that utilizes a laser frequency splitting technique. To avoid strong mode competition in measuring half and full wave plates, we use two separate methods: comparing adjacent longitudinal mode spacing, and phase offset with an additional quarter wave plate. Therefore any wave plate can be characterized by a single instrument, and no complicated experimental arrangement or data analysis is required. The performance of the system is demonstrated by determining the phase retardation of several samples to a precision and repeatability better than lambda/10(4); moreover, an error analysis is proposed.

Optical feedback laser with a quartz crystal plate in the external cavity

We studied the optical feedback characteristics of a single-mode He-Ne laser with a quartz crystal plate in the external cavity. The fringe frequency of the laser system can be doubled when the quartz crystal plate is positioned at a certain angle between the crystalline axis and the beam in the crystal plate. Theoretical analysis shows that the birefringent effect of the quartz crystal plate and the laser beams second pass through the external cavity result in this phenomenon. The experimental results are in good agreement with the theoretical analysis. A quartz crystal plate can double the resolution of a self-mixing sensing system.

Self-mixing interference effects of microchip Nd:YAG laser with a wave plate in the external cavity

We present the optical feedback characteristics of a single-mode Nd:YAG laser with a wave plate in the external cavity. The laser intensities of the two orthogonal directions, which are both modulated by the change of external cavity length, have a phase difference due to the birefringence effect of the wave plate. When threshold intensity is introduced, a period of intensity fringe can be divided into four equal zones. Each zone corresponds to lambda/8 displacement of the external feedback reflector. The direction of displacement can be discriminated by the sequence of these four zones. This phenomenon provides a potential displacement sensor with directional discrimination and high resolution of eighth wavelength compared with the traditional optical feedback.

Analysis of the effects of feedback asymmetry in external cavity He-Ne lasers

A theoretical analysis of the characteristics of an external cavity He-Ne laser with asymmetric feedback induced by the misalignment of an external feedback mirror is proposed. The theoretical model considers both the multiple reflections and the phase variation caused by the asymmetric external cavity. It is found that the phase variation is of importance in determining the effects of asymmetric feedback. The three-order asymmetric feedback effects are experimentally observed and can be interpreted well by this model. The fringe frequency of the asymmetric optical feedback system can also be increased. The experimental results are in good agreement with the theoretical analysis. The theoretical and experimental results offer a potential increase in the resolution of an optical feedback system with asymmetric feedback.

Laser frequency splitting method for high-resolution determination of relative stress-optic coefficient and internal stresses in Nd:YAG crystals

We propose a laser frequency splitting method to determine the relative stress-optic coefficient and internal stresses in Nd:YAG crystals with high resolution. In this method a mirrored Nd:YAG crystal in a circular disk shape is made into an all-internal cavity laser. Once diametral compression force is applied, due to intracavity photoelastic effect, the single laser frequency is split in two. Through measuring the beat frequency of the split frequencies, the relative stress-optic coefficient and internal stresses in the Nd:YAG crystal can be determined according to the proportional relationship. In our experiment the measured value of the stress-optic coefficient, C(111)=1.27x10(-12) m(2)/N, is close to the theoretical value; the principal stress difference at the center of the Nd:YAG crystal was also determined. The optical path retardation resolution is approximately 1.5x10(-8) lambda, and correspondingly the principal stress difference resolution is approximately 5 Pa. The resolution is approximately 5-8 orders of magnitude higher than those of conventional methods, therefore even very small residual stresses can be easily determined with this method. Although this method is a kind of point-by-point method instead of a full-field method, it is a promising novel photomechanics method with attractive advantages such as extremely high resolution.

Influence of external cavity length on multimode hopping in microchip Nd:YAG lasers

The influence of external cavity length on multimode hopping in microchip Nd:YAG lasers is investigated experimentally. With an optical feedback loop, the threshold gain of different longitudinal modes are all modulated by changing the external cavity length; a lambda/2 change in the external cavity length causes a one-period oscillation. The longitudinal modes can be divided into groups according to different initial threshold gain variations and modulation trends corresponding to different external cavity phases. Because of the initial gain difference, only one mode in each group is the dominant potential lasing mode, while others are suppressed. During the 2 pi change of the external cavity phase, mode hopping occurs among these potential lasing modes from different groups. Both the intensity waveforms and the number of hopping modes strongly depend on the external cavity length. Experimental results agree well with the theoretical analysis of the phenomenon of multimode hopping subjected to optical feedback in microchip Nd:YAG lasers.

Heterodyne wavelength meter for continuous-wave lasers.

A wavelength meter based on a heterodyne interferometer is presented. A single-wavelength test laser beam is modulated to two orthogonal linearly polarized components with different frequencies by a pair of acousto-optic modulators. Then the modulated laser beam and a two-wavelength laser beam are sent to a heterodyne interferometer in a common path. The ratio of two laser interference phase shifts in the heterodyne interferometer is equal to the ratio of their wavelengths. The heterodyne technique measures the heterodyne interference phase but not the interference intensity, which means that it could measure a light source whose intensity is not stable. The heterodyne interference signal is an alternating signal that can easily magnify and process the circuit that makes up the heterodyne wavelength meter and could be used to measure the low-intensity light source even when there are environmental disturbances. A tunable diode laser wavelength range of 630-637 nm has been measured to an accuracy of 5 parts in 10(7).

High-frequency intensity modulation in orthogonal polarized dual frequency lasers with optical feedback.

High-frequency modulation of laser output intensity is studied with asymmetric feedback induced by the misalignment of an external feedback reflector in an orthogonal polarized dual frequency laser. The fringe frequency of the optical feedback system is seven times higher than that of a conventional optical feedback system, due to multiple feedback effects. The output characteristics of two orthogonal polarized modes are also investigated. Mode competition is observed between the two modes. When initial intensities of the two modes are unequal, the mode competition will be strong. The difference in initial intensity between the two orthogonally polarized modes plays an important role in the mode competition with optical feedback. Experimental results are presented, as well as a theoretical explanation. The high-frequency modulation of laser intensity can greatly increase the resolution of an optical feedback sensing system.

Effects of optical feedback in a birefringence-Zeeman dual frequency laser at high optical feedback levels

Optical feedback effects are studied in a birefringence-Zeeman dual frequency laser at high optical feedback levels. The intensity modulation features of the two orthogonally polarized lights are investigated in both isotropic optical feedback (IOF) and polarized optical feedback (POF). In IOF, the intensities of both beams are modulated simultaneously, and four zones, i.e., the e-light zone, the o-light and e-light zone, the o-light zone, and the no-light zone, are formed in a period corresponding to a half laser wavelength displacement of the feedback mirror. In POF, the two orthogonally polarized lights will oscillate alternately. Strong mode competition can be observed, and it affects the phase difference between the two beams greatly. The theoretical analysis is presented, which is in good agreement with the experimental results. The potential use of the experimental results is also discussed.