双频激光技术研究新进展及发展趋势

双频激光器在干涉测量和光学传感等技术领域扮演极其重要的角色。综述了近年来双频激光技术研究的新进展(包括双折射双频He Ne激光器、双折射双频Nd:YAG激光器和双纵模半导体激光器),简要介绍了各种双频激光器的工作原理,分析了这些双频激光器在频差大小、频差稳定性和频差调谐特性等方面的优缺点,指出了今后双频激光技术研究的发展趋势。     

HeNe双频激光器频差的激光内雕赋值法

在精密测量领域,HeNe激光器是制造激光干涉仪的首选.但能够产生中频差4—40 MHz的双折射-塞曼双频HeNe激光器的频差是随机的,小到几百kHz,大到十几MHz.为了让频差能够满足制作双频激光干涉仪的要求,曾出现过弹性加力法、钻孔应力调节法等调整和频差赋值的方法.但这些方法在实际应用中都不是很理想.采用激光内雕法赋值频差,通过在激光器的平面输出镜内部雕刻出不同图案,使平面镜内部产生满足要求的相位延迟,达到为频差赋值的目的.激光内雕法有很多优点:不损害激光器,美观;功率损失小,出光强度可接近原始光强度;可反复赋值,频差在0—60 MHz范围内可调整;频差的稳定性好,采用稳频措施后频差的波动范围小于每小时10 kHz.     

用于光生毫米波的双频微片激光器热致频差特性研究

研究了激光二极管端面抽运的双纵模双频微片激光器的热效应对其输出频谱的影响. 通过对端面抽运Nd:YVO₄微片的热传导方程求解得出晶体内部的温度场分布; 并进一步对微片晶体热致折射率变化进行分析, 由此计算出双频微片激光器的热致频谱变化. 实验结果表明: 当抽运功率较小时得到稳定的双纵模双频激光输出; 当抽运功率增大时, 晶体热效应使双频激光频谱双峰之间的频差减小, 各纵模波长频谱宽度加宽, 实验结果与理论分析相符合. 关键词： 光载无线通信 双频微片激光器 热效应 频率差     

Beam combination setup for dual-frequency laser with orthogonal linear polarization

A beam combination setup for a dual-frequency laser with orthogonal linear polarization is proposed. It consists of two polarizing beam splitters(PBSs) whose polarization axes are orthogonal to each other. A theoretical analysis demonstrates that a combined dual-frequency laser beam with this setup strictly meets orthogonal linear relation. The experimental results show that compared with the conventional setup, the ellipticity and nonorthogonality of the combined dual-frequency laser beam are significantly reduced.     

Effect of slit dual-frequency ultrasonic emulsification technology on the stability of walnut emulsions

A highly hygienic walnut emulsion beverage was prepared by using a slit dual-frequency emulsification technique. The optimal ultrasonic parameters were studied as a model system: the ultrasonic time of 50 min, the ultrasonic power density of 260 W/L, and a dual-frequency ultrasonic combination of 28/68 kHz. Walnut emulsion with an average mean volume diameter of 2.05 µm, a Zeta potential absolute value of 40 mV was obtained after ultrasonic treatment, and the emulsion stability could be maintained for more than 14 days without phase separation. At the lowest ultrasonic energy input, the vibrating emulsion could promote droplet aggregation. However, excessive energy input could result in sample overtreatment and reduced emulsion activity. The laser scanning confocal microscope (LSCM) and transmission electron microscope (TEM) confirmed that walnut emulsion processed by slit dual-frequency ultrasonic had pretty high stability. Therefore, the slit dual-frequency ultrasonic emulsification technique was found to be well suited for the preparation of complex and fine oil-in-water food emulsions.     

高功率宽带射频调制连续激光源

射频强度调制激光作为激光雷达系统的载波可以有效提高系统的抗干扰和抗散射能力,高功率宽带射频强度调制光源是实现高分辨率远距离探测的关键.本文采用在Nd:YAG激光器的耦合腔中插入一对四分之一波片的方法实现了频差调谐范围为30 MHz—1.5 GHz的双频激光输出,结合光纤振荡功率放大技术,将双频信号光功率放大为50 W.耦合腔双频种子源具有良好的功率和频率稳定性,输出功率为9.5 mW时,功率标准差为0.145 mW,稳定性为1.52%,输出双频激光的频差为250 MHz时,拍频的标准差为1.6144 MHz.种子光进行三级光纤功率放大,得到50 W双频激光输出.放大后的双频激光功率波动范围小于0.1 W,双频拍频的标准差为1.777 MHz,很好地保持了放大之前的功率稳定性和双频频差稳定性.     

半外腔微片Nd∶YAG正交偏振激光器及其在精密测角中的应用

设计了一种半外腔微片Nd∶YAG正交偏振双频激光器。把2个1/4波片置于激光谐振腔内,一个静止,一个作360°旋转,旋转引起2个1/4波片快轴之间的角度变化被激光器转化成激光2个频率之差的变化,从而输出可调谐的双频激光。采用琼斯矩阵对光在腔内的本征模进行分析,给出了双频频差的理论解释。讨论了一种体积小、分辨率高和可整周测量的新型Nd∶YAG激光测角仪的潜在前景。     

Two kinds of novel birefringent dual-frequency lasers

Two kinds of novel dual-frequency He–Ne lasers are presented and described in this paper based on the birefringent laser frequency splitting technology: the full-inner cavity photoelastic dual-frequency laser and the half-inner cavity photoelastic dual-frequency laser. The frequency difference between the two kinds of dual-frequency laser can be adjusted from tens to hundreds of megahertz through adjusting the mechanical pressure exerted on the photoelastic element. The stability of the frequency differences of the two lasers are 10⁻³ and 10⁻⁵, respectively, after the lasers are turned on for a while.     

Frequency difference lock-in phenomenon's weakening by transverse magnetic field in Y-shaped cavity dual-frequency laser

We report experimental progress in weakening the frequency difference lock-in phenomenon in a Y-shaped cavity dual-frequency laser. A cube coil pair is chosen to provide a uniform magnetic field for tunability and uniformity of magnetic field strength. When the transverse magnetic field intensity is 9 m T, the frequency difference lock-in phenomenon is evidently weakened and the frequency difference can be continuously tuned in the range of0.12 MHz to 1.15 GHz. Moreover, the relationship between the minimal frequency difference and magnetic field intensity are investigated and discussed. Then a Y-shaped cavity dual-frequency laser is expected to be utilized as an optimum light source for heterodyne interferometric sensing and precise laser measurement.     

Laser drilling of thick material using femtosecond pulse with a focus of dual-frequency beam

Laser drilling is one of the basic, most frequently performed, material removal processes. The drilling aspect ratio is theoretically limited by the size and the focal depth of the machining laser spot. The aspect ratio can be improved by using dual focus. In this paper we describe a focus of two different frequencies based on the longitudinal chromatic aberration arisen when polychromatic collimated light is incident on a positive lens element. In the experiments, a Ti:Sapphire laser of 800 nm wavelength and 150 fs pulse duration was used as a source. Two tightly focused laser spots few hundred micrometers apart from each other were formed by focusing a combined collimated laser beam which contains the fundamental optic frequency and the second harmonic optic frequency. The focus of dual-frequency beam was used to drill a 3 mm thick PMMA plate. The drilling aspect ratio of a dual-frequency beam was compared to that of a focus of single frequency beam. Experimental results reveal that dual-frequency beam increases the aspect ratio and improves the drilling quality in terms of profile of the produced features.     

Diode-pumped Birefringent Tunable Nd:YAG Laser with Large Frequency Difference

A dual-frequency Nd: YAG laser which can output two frequencies with frequency difference of more than 5 gigaherz is developed. By inserting and moving an intracavity quartz crystal wedge in diode pumped Nd:YAG laser, It can be obtained and tuned frequency difference in a wide frequency range. The stabilization of frequency difference in free state is much better than that of birefringent dual-frequency He-Ne lasers at similar laboratory conditions. With such a large tunable frequency difference i.e. short synthetic wavelength, the laser may be used as the light source of absolute distance interferometers. In our experiment, the methods for tuning frequency difference by rotating and applying a tunable force on the intracavity quartz plate are also studied.     

微片Ｎｄ∶ＹＡＧ 双频激光器腔调谐现象研究

 基于微片Ｎｄ∶ＹＡＧ 正交偏振双频激光器,研究了若干重要的双频激光器腔调谐现象, 包括光强调谐、频差调谐、子谐振腔效应及频差闭锁等,给出了实验结果和数据。腔调谐下,正交双频的频差调谐量约为３５０ ｋＨｚ;存在子谐振腔效应时,频差调谐量最大可达到２ ＭＨｚ;未发现明显频差闭锁现象,频差最小值可达到１４ ＭＨｚ。     

Study of Hard-sealed,Tunable,Photoelastic Birefringent Dual-frequency Lasers

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Precise force measurement method by a Y-shaped cavity dual-frequency laser

A novel precise force measurement based on a Y-shaped cavity dual-frequency laser is proposed.The principle of force measurement with this method is analyzed,and the analytic relation expression between the input force and the change in the output beat frequency is derived.Experiments using a 632.8-nm Y-shaped cavity He-Ne dual-frequency laser are then performed;they demonstrate that the force measurement is proportional to a high degree over almost five decades of input signal range.The maximum scale factor is observed as 5.02×109 Hz/N,with beat frequency instability equivalent resolution of 10-5 N.By optimizing the optical and geometrical parameters of the laser sensor,a force measurement resolution of 10-6 N could be expected.     

Diode-pumped Birefringent Tunable Nd: YAG Laser with Large Frequency Difference

A dual-frequency Nd: YAG laser which can output two frequencies with frequency difference of more than 5 gigaherz is developed. By inserting and moving an intracavity quartz crystal wedge in diode pumped Nd:YAG laser, It can be obtained and tuned frequency difference in a wide frequency range. The stabilization of frequency difference in free state is much better than that of birefringent dual-frequency He-Ne lasers at similar laboratory conditions. With such a large tunable frequency difference i.e. short synthetic wavelength, the laser may be used as the light source of absolute distance interferometers. In our experiment, the methods for tuning frequency difference by rotating and applying a tunable force on the intracavity quartz plate are also studied.     

Beat-note jitter suppression in a dual-frequency laser using optical feedback

An efficient locking technique based on optical feedback is demonstrated to suppress jitter on the rf beat note between the two modes of a dual-frequency Yb:Er glass laser. The method consists of a self-injection process in which one selected mode serves as a master oscillator to lock and stabilize the second mode via a frequency-shifted optical feedback. The beat note adjusted near 170 MHz was stabilized with an accuracy of 250 mHz using an optical feedback loop with a double pass through an acousto-optic modulator. The beating note can be tuned over 300 kHz by controlling the reference oscillator. The extensions and limitations of the technique are discussed.     

A novel optical subdivision method for dual-frequency interferometer

The theory of optical subdivision techniques of dual-frequency laser interferometers is stated. And a novel optical subdivision technique is proposed originally to enhance resolution of a commercial interferometer by adding some corner-cubes. Then the performance of the interferometer is tested. The interferometer resolution of 1.24 nm and the average error of below 2 nm are achieved by using the technique. The most novel of the optical subdivision technique is without λ/4 plates. It is less sensitive to environmental changes, it has prodigious potential to improve resolution farther and it can reduce polarization mixing error.     

基于双频光源和涡旋相位的互补旋转集束匀滑方案

提出了一种基于双频光源和涡旋相位板实现光束快速互补旋转的集束匀滑方案。双频光源为集束中各子束提供频移,拓扑荷数相同但反号的涡旋相位板阵列用于将各个子束变换成拉盖尔—高斯(LG)光束,而通过偏振控制则可实现子束间两两的相干叠加。在此基础上,通过采用共轭连续相位板可使波长不同、偏振态不同的子束组合在靶面形成快速旋转且空间上互补填充的焦斑。结果表明,利用这一方案可实现子束散斑在靶面上快速旋转且散斑分布保持互补,进而有效改善靶面辐照均匀性,甚至为抑制激光等离子体不稳定性提供了一种潜在途径。     

A 5 MHz beat frequency He–Ne laser equipped with bireflectance cavity mirror

A He–Ne dual-frequency laser equipped with a bireflectance cavity mirror is described. It is based on the principle that when the incident light is perpendicular to the bireflectance cavity mirror, the phase delays of the reflective light along the mirror's fast and slow axes are different because of the photoelastic effect. The mode coupling is reduced by utilizing transverse Zeeman effect so that two linear and orthogonal polarization components with 5 MHz beat frequency are generated. The effect of the magnetic field's direction on the dual-frequency as well as the polarization property of the laser are investigated by experiments. After stabilizing the frequency, the laser is calibrated with the iodine frequency stabilization laser of Chinese National Institute of Metrology (CNIM). Experimental results indicate that the expanded uncertainty of wavelength in vacuum is 1×10⁻⁷ (k=3) with a frequency stabilization of 6.6×10⁻¹⁰ (sampled in 1000 s).     

Dual-frequency paired polarization phase shifting ellipsometer

A novel dual-frequency paired polarization phase shifting ellipsometer (DPPSE) is proposed and experimentally demonstrated. It combines the features of the phase shifting interferometer and common-path polarized heterodyne interferometric ellipsometer where the ellipsometric parameters (EP) of a specimen are measured accurately. The experimental results verify that DPPSE is capable of determining the full dynamic range of EP. In addition, the properties of dual-frequency paired linearly polarized laser beam in DPPSE perform in common phase noise rejection mode. It is insensitive to environmental disturbance and laser frequency noise. The capability of DPPSE to perform higher accuracy EP measurement than conventional ellipsometer is verified according to error analysis.