External-cavity birefringence feedback effects of microchip Nd:YAG laser and its application in angle measurement

External-cavity birefringence feedback effects of the microchip Nd:YAG laser are presented. When a birefringence element is placed in the external feedback cavity of the laser, two orthogonally polarized laser beams with a phase difference are output. The phase difference is twice as large as the phase retardation in the external cavity along the two orthogonal directions. The variable extra-cavity birefringence, caused by rotation of the external-cavity birefringence element, results in tunable phase difference between the two orthogonally polarized beams. This means that the roll angle information has been translated to phase difference of two output laser beams. A theoretical analysis based on the Fabry--Perot cavity equivalent model and refractive index ellipsoid is presented, which is in good agreement with the experimental results. This phenomenon has potential applications for roll angle measurement.     

Anisotropic optical feedback of single frequency intra-cavity He--Ne laser

This paper presents the anisotropic optical feedback of a single frequency intra-cavity He--Ne laser. A novel phenomenon was discovered that the laser output an elliptical polarized frequency instead of the initial linear polarized one. Two intensities with a phase difference were detected, both of which were modulated in the form of cosine wave and a fringe shift corresponds to a λ/2 movement of the feedback mirror. The phase difference can be continuously modulated by the wave plate in the external cavity. Frequency stabilization was used to stabilize the laser frequency so as to enlarge the measuring range and improve the measurement precision. This anisotropic optical feedback system offers a potential displacement measurement technology with the function of subdivision of λ/2 and in-time direction judgment. The three-mirror Fabry--Perot cavity model is used to present the experimental results. Given the lack of need of lasing adjustment, this full intra-cavity laser can significantly improve the simplicity and stability of the optical feedback system.     

Generation and Modulation of Phase Difference of Output Intensities in a Feedback Nd:YAG Laser with an Extracavity Waveplate Rotated

External anisotropic feedback effects on the phase difference behaviour of output intensities in a microchip Nd：YAG laser are presented. By rotating a quarter wave plate placed in the external cavity, the angle between laser initial polarization direction and o-axis of the wave plate is tuned from -45°to 45°, which results in variable extra-cavity birefringence along two orthogonal detection directions. With only one optical path and one wave plate, laser intensities of the two orthogonal directions, both modulated by the external cavity length, are output with a tunable phase difference, which can be continuously changed from zero to twice as large as that of the waveplate. Experimental results as well as a theoretical analysis based on Fabry-Perot cavity equivalent model and the refractive index ellipsoid, are presented. The potential applications of this phenomenon are also discussed.     

Intensity tuning characteristics of double-mode He--Ne laser with optical feedback

This paper investigates the intensity tuning characteristics of a double longitudinal modes He--Ne laser subjected to optical feedback. The intensity undulations of the total light and the two modes are observed for different external cavity length. Two modulations of the internal cavity length are performed. One is only for the internal cavity length being modulated and the other is for both the internal and the external cavity length being modulated. The undulation frequency of the total light is found to be determined by the ratio of external cavity length to internal cavity length in both modulations. When the external cavity length is integral times of the internal cavity length, the fringe frequency of the total light could be seven or even more times of that in conventional optical feedback. A simple theoretical analysis is presented, which is in good agreement with the experimental results. The potential use of the experimental results is also discussed.     

各向异性外腔微片Nd:YAG激光器调频回馈研究

研究了在各向异性外腔下微片Nd:YAG激光器调频回馈特性.作为对比,首先给出了在各向同性外腔下调频回馈结果,激光器输出功率的半波条纹数正比于外、内腔长度之比.在回馈外腔中内置一个四分之一波片,激光偏振态发生规律性跳变,每个纵模带宽内的偏振跳变次数正比于外、内腔长之比,相同外腔长下,回馈条纹再次倍频.进行了基于法布里-珀罗腔等效模型的理论分析,结果和实验相符合.此现象在精密测量领域有潜在应用. 关键词： 光回馈 调频回馈 各向异性外腔 半波条纹数     

调频微片Nd:YAG激光器光回馈特性的研究

研究了调频微片Nd：YAG激光器的光回馈特性.在没有光回馈的情况下，激光频率每改变一个纵模间隔，激光器输出功率变化一个条纹.有光回馈时，同时改变激光频率和外腔相位，激光器输出功率波动频率正比于内、外腔长度之比.基于三镜腔等效模型的理论分析和实验结果相符合.并且讨论了这一现象在精密测量领域的潜在应用.     

Optical feedback in a Zeeman-birefringence HeNe laser and its application in ranging

The optical feedback characteristics in a Zeeman-birefringence HeNe laser have been studied systematically when the external cavity varies in a large range. The intensity variations of the two orthogonally polarized lights are observed during the laser cavity tuning in the dual frequency laser. When the external cavity length is an integral multiple of the laser cavity length, either polarized light's intensity modulation depth varies not obviously when the laser works in dual modes conditions or single mode condition. But when the external cavity length is far away from an integral multiple of the laser cavity length and the laser works in different dual modes conditions, polarized light's intensity modulation depth varies obviously. This characteristic can guide us to tune the laser to work in single mode condition to improve the sensitivity of the optical system when the system is used for a large displacement measurement. It can also be used for measuring the position of the target mirror. The theoretical analysis is in good agreement with experimental results.     

光纤激光器自混合干涉效应研究

针对光纤激光器自混合干涉传感应用,研究了光纤激光器自混合干涉特性,运用四镜法布里-珀罗腔模型对掺铒线形腔光纤激光器自混合干涉效应进行了理论分析,对不同反馈水平下的自混合干涉信号进行了数值模拟,获得了光反馈条件下光纤激光器输出特性.外腔长度的改变会调制激光器的输出强度,外腔长度变化半个波长,对应一个干涉条纹,弱反馈条件下,由反馈引起的激光器的频率变化可以忽略.设计了基于光纤激光器的自混合干涉实验,实验结果和理论分析相符合.此研究结果为进一步开展光纤激光器的自混合干涉传感应用研究奠定了理论与实验基础.     

Polarization switching in a quasi-isotropic microchip Nd:YAG laser induced by optical feedback

This paper demonstrates the influence of external optical feedback on the polarization state of longitudinal modes in quasi-isotropic microchip Nd:YAG lasers. Under optical feedback, the polarization state of longitudinal modes in quasi-isotropic lasers relies strongly on the intracavity anisotropy loss and mode competition. When the intracavity anisotropy loss is small, external optical feedback can cause polarization switching and strong mode competition between two orthogonal linearly polarized eigenstates of one laser longitudinal mode, which leads to the distortion of laser intensity modulation waveform. The polarization switching is independent of the initial external cavity length. By increasing the intracavity anisotropy loss, one polarization eigenstate can be suppressed and the laser works in single-polarization state. A theoretical analysis based on the compound cavity model is presented, which is in good agreement with the experimental results. The results offer guidance to the development of laser feedback interferometers.     

Single high-order round-trip feedback effects in orthogonally polarized dual frequency laser

We present the effects of single high-order round-trip feedback in orthogonally polarized dual frequency laser. The high quality intensity fringes of two orthogonally polarized lights are obtained, which are cosine-like. The fringe frequency of a single high-order round-trip feedback is five times that of the conventional optical feedback. Even higher frequencies are possible with this setup. Particularly, there is a phase difference between two orthogonal polarized lights in single high-order round-trip feedback and the phase difference is determined by the feedback order, the length of the external cavity, and the frequency difference of the two orthogonal polarized lights. A theoretical analysis based on a compound cavity model of a dual frequency laser agrees well with the experimental results. These results provide a potential application in precision measurement with both directional discrimination and high optical resolution.     

Coherent beam combination of broad-area laser diode array using off-axis external cavity with double feedback

In this letter, we demonstrate coherent beam combination of laser diode array using the external Talbot cavity with double feedback. The double feedback elements consist of grating and high-reflection plane mirror. Compared with single high-reflection plane mirror feedback, the external Talbot cavity with double feedback reduce the number of interference strips in the far-field pattern and narrow spectral line-width of the laser diode array. The results indicate that the application of the external Talbot cavity with double feedback produces a clear far-field interference pattern. In addition, line-width is reduced to 0.15 nm full-width at half-maximum (FWHM).     

闪耀光栅外腔反馈压窄半导体激光器线宽技术的研究

在讨论半导体激光线宽压窄理论的基础上,利用闪耀光栅作为外部反馈元件,介绍了由中心波长为949.6nm、原始线宽为1.2THz的单管半导体激光器构成的反馈外腔,它能够很好的改善半导体激光器的性能。实验得到了中心波长稳定的、单纵模的高质量激光输出,边模抑制比大于30dB,线宽优于1.2MHz(Δλ<3.6×10-6nm)。实验证实了强反馈能够很好地改善外腔半导体激光器的动态特性。     

Observation of multi-path interference in broad-area semiconductor lasers with optical feedback

Multi-path interference effects induced by optical feedback in broad-area semiconductor lasers is experimentally studied. An external mirror and an internal laser cavity form a closed composite optical feedback loop. For a very small tilt of the external mirror for the exit facet along the stripe width of the active layer, the light emitted from the laser undergoes multi-path reflections in the composite cavity, and we therefore observe multi-path interference effects of the laser oscillations for a small mirror tilt. The laser output power always shows the fundamental mode of the single feedback loop together with an oscillation of a certain higher multi-path loop. The laser oscillation and the beam profile are strongly dependent on the tilt. We observed up to a nine-fold multi-path interference in the experiment.     

Self-mixing interference in distributed feedback laser diode with multiple external cavities

The self-mixing interference in distributed feedback laser diode (DFB-LD) with multiple external cavities is analyzed. Each external cavity is considered to be an optical thin film, and the equivalent reflectivity can be got from the theory of the thin film optics, the general expressions of gain and frequency in compound laser cavity are theoretically deduced. This principle is helpful to build the fiber-coupled self-mixing interference system. Considering that different parameters have influence on self-mixing interference, we make some simulation analysis at different conditions. The output of self-mixing interference is analyzed in numerical analysis, and the amplitude variations of the output gain is discussed along with different parameters, e.g., the coupling coefficient, the linewidth enhancement factor, and the reflection coefficient of external reflector.     

Self-mixing interference effects in orthogonally polarized dual-frequency Nd:YAG lasers at different feedback levels

The self-mixing interference effects are investigated in birefringence dual-frequency Nd:YAG lasers at different optical feedback levels. For weak feedback level, both of the intensities of the two modes are sinusoidally modulated by external cavity length with a feedback period of half laser wavelength. While these two modes can only coexist in a part of feedback period at moderate feedback level, i.e., one mode reaches its maximum intensity, the other one extinguishes. At high feedback level, complete polarization switching (one mode oscillating, the other one extinguishing) between two polarized modes is observed, and every switching corresponds to quarter-wavelength variation of external cavity length. Furthermore, two feedback schemes, i.e., isotropic optical feedback (IOF) and polarized optical feedback (POF) are taken into account. The theoretical analysis based on a model of two-mode-coupled laser is presented, which is in good agreement with experimental results. Our results can advance the research of self-mixing interferometer of orthogonally polarized dual-frequency lasers.     

存在外部光反馈的半导体激光器的相位噪声

本文在理论上研究了由于外部光反馈引起的半导体激光器的相位噪声.理论分析是通过引入一个反馈耦合率K和线性化速率方程,导出了半导体激光器相位噪声功率谱密度的表达式.这表明相位噪声功率谱密度随外腔长度而周期性地漂移,且功率谱密度的峰值随外部反馈耦合率而发生很大的变化.     

External anisotropic feedback effects on the phase difference behavior of output intensities in microchip Nd:YAG lasers

We present the effects of optical feedback from an anisotropic external cavity on the characteristics of the output intensities in a single mode Nd:YAG laser. Due to the birefringence effect, the external cavity modulates laser intensities in the two orthogonal directions with a phase difference, which is twice as large as that of the birefringence element in the external cavity. Experimental results are presented, as well as a theoretical analysis. The potential applications of this phenomenon are also discussed. PACS 42.60.Jf; 42.62.Eh; 42.25.Lc; 42.60.By     

双反馈半导体激光器的混沌特性研究

实验利用双反馈半导体激光器获得了关联维数为3.8的高维混沌光.同时对比分析了双反馈与单反馈两种不同模式产生混沌的区别.结果表明：在反馈强度均为-26 dB时,双反馈产生混沌的关联维数高于单反馈产生混沌的关联维数2.6;双反馈可获得带宽为11 GHz的混沌光,为单反馈产生混沌带宽5.5 GHz的两倍.当双反馈的两个外腔长度不相等时,混沌的自相关曲线能很好的隐藏外腔长度信息,可提高混沌通信的保密性. 关键词： 双反馈 半导体激光器 混沌 带宽     

Study on a broad-area laser diode with external cavity feedback

The external cavity feedback of a broad-area laser diode is investigated. In Fourier optic methods, the inserted prism in an external cavity can be interpreted as a spatial filter in the Fourier plane of the laser diode. The transverse mode can be selected by changing the position of the filter. As the result of a self-consistency theoretical analysis, the output beam of the external cavity has a single-lobed far-field profile. It agrees well with the experiment result. The beam times-diffraction-limit-factor M² is measured to be 1.16.     

Laser Intensity Variation in Amplitude and Phase Induced by Elliptically Polarized Feedback

The laser output characteristics under elliptically polarized optical feedback effect are studied. Elliptically polarized light is generated by wave plate placed in the feedback cavity. By analyzing the amplitude and phase of the laser output in the orthogonal direction, some new phenomena are firstly discovered and explained theoretically.Elliptically polarized feedback light is amplified in the gain medium in the resonator, and the direction perpendicular to the original polarization direction is easiest to oscillate. The laser intensity variation in amplitude and phase are related to the amplified mode and the anisotropy of external cavity. The theoretical analysis and experimental results agree well. Because the output characteristic of the laser has a relationship with the anisotropy of the external cavity, the phenomenon also provides a method for measuring birefringence.