For precision measurement in an interferometer setup very often He-Ne laser tubes are used which are frequency stabilized to increase the coherence length. Usually, this is done by wrapping a heater around the tube to control the resonator length. For this purpose, short tubes are used (in the <2 mW range) which have only two longitudinal modes. These modes are perpendicular polarized to each other. A common approach to get the control signal for the heater is to separate the two modes by a polarizing beam splitter and to direct them on two separate detectors. A control electronics drives the heater in a way that the difference signal of the two detectors is kept zero.
In some cases, the two modes instantaneously switch their polarization state. This leads to a loss of locking at the control electronics. Tubes were this happens are useless for this application. This phenomenon is called “polarization switching” sometimes, though not quite correct, “mode hopping”.
This has been a big problem as this behavior is not predictable and lasers showing no polarization switching had to be selected with varying yield. To overcome this, LASOS developed a new manufacturing method allowing to suppress the polarization switching precisely and permanently. This development allows also to suppress the negative effect of weak magnetic fields on the polarization switching process.
LASOS is therefore able to offer He-Ne lasers with the guarantee of no polarization switching. Together with the excellent beam quality and the long lifetime they are the perfect choice for high performance instruments.