stabilized Nd:YAG based two-wavelength-two-frequency-lasers for refraction corrected length measurement

对于许多干涉测量任务，空气折射率的补偿要么目前不太可能，要么需要不成比例的高技术设备。近年来，双波长法一直是研究的热点。然而，由于没有合适的激光源，到目前为止还不可能使其适用。本应用旨在通过使用Nd:YAG激光器，结合各种物理方法来产生两个频率的两个波长，从而以一种简单的方式实现良好的稳定，从而消除这种缺陷。到目前为止，人们已经深入研究了Nd:YAG晶体中的光弹性效应，并将其应用于线性超过12年的力传感器中。相反，该应用程序的另一个主要焦点是提供由压电力应用产生的正交偏振激光模式的恒定拍频。因此，在第二步中，这两种模式可以用于气体吸收电池的不饱和吸收峰上的双模式稳定，因为这种情况已经在很长一段时间内用于HeNe激光器。这种频率稳定显然比目前应用的边缘技术有更好的相对和绝对稳定性（目标：更好的10-9）。此外，所提出的方法比饱和吸收线上的稳定方法要简单得多。在这里，机械和光学耦合闭环控制系统与节拍和绝对频率调节的必要解耦是一项特别高的科学挑战。力控拍频是实现双模稳定的先决条件，现在可以直接用于外差技术的干涉仪。利用KTP晶体的倍频技术，在Nd:YAG激光器中已经应用了很长时间，最终使波长稳定在532 nm（这是双波长法所必需的），同步到1064 nm，并且拍频也自动调节。该项目的目标是为实现低成本稳定的两波长两频率激光器创造科学技术基础，该激光器可以很好地用于工业测量任务，其绝对频率不确定度为10-9。这些激光器可能适用于折射率补偿干涉长度测量的各种应用，特别是用于抑制长基线干涉测量中发生的空气湍流的影响。

For many interferometric measuring tasks, a compensation of the refractive index of air is either not sufficiently possible at present or would require a disproportionately high amount of technical equipment. In recent years, the two wavelength method has been in the centre of intensive investigations. However, it has not been possible so far to make it applicable because of the unavailability of a suitable laser source.This application aims to eliminate this deficit by combining various physical approaches for generating two wavelengths of two frequencies each, using a Nd:YAG laser thus allowing good stabilization in an easy way. So far, the photoelastic effect occurring in Nd:YAG crystals has been examined intensively for the application as force sensor with a linearity over 12 decades.Conversely, another main focus of this application is to provide a constant beat frequency of the orthogonally polarized laser modes resulting from the application of a piezoelectric force. Thus, in the second step, these two modes can be used for two mode stabilization in this case on an unsaturated absorption peak of a gas absorption cell as it has been the case with HeNe lasers for a long time already. This kind of frequency stabilization promises clearly better relative and absolute stabilities (goal: better 10-9) than the side of fringe technology applied so far. Also, the approach proposed is considerably simpler than stabilization methods on saturated absorption lines.Here, the necessary decoupling of the mechanically and optically coupled closed loop control system from the beat and absolute frequency regulation is a task representing a particularly high scientific challenge. The force controlled beat frequency, which is the prerequisite for the 2 mode stabilization, can now be used in the interferometer directly for the heterodyne technique. The frequency doubling by means of KTP crystal, as it has been used with the Nd:YAG lasers for a long time already, finally results in the stabilized wavelength at 532 nm (which is necessary for the two wavelength method), synchronously to the 1064 nm, with the beat frequency being regulated automatically, too. The goal of this project is to create the scientific technological bases for realizing low cost stabilized two wavelength two frequency lasers, which can well be used for industrial measuring tasks, presenting an absolute frequency uncertainty of 10-9. These lasers should potentially be suitable for a variety of applications of refractive index-compensated interferometric length measurement, in particular for suppressing the effect of air turbulences occurring in long baseline interferometry.