The PicoScale allows to perform high precision displacement measurements over a large working range. PicoScale’s signal-to-noise ration is best if the user defines the expected working range as precise as possible. In this application note we describe, what working ranges are possible for given working distances, and vice versa.
Introduction and Definitions
The performance of the PicoScale laser interferometer depends much on the distance of the sensor heads to the object mirror. On one hand, keeping the optical path length as short as possible minimizes the beam divergence and the influence of external disturbances. These include air movement, spatial and temporal gradients in environmental parameters such as pressure, temperature and humidity, and vibrations of the optical setup.
On the other hand, due to the operating principle of the PicoScale, the amplitudes of the two quadrature signal components (Sw and S2w) depend on the distance between mirror and sensor head. This is shown schematically in figure 1. A reliable measurement is possible only when the amplitudes of both signal components are above the indicated threshold. When larger working distances are set in the PicoScale GUI, the period lengths of these functions increase. This in turn allows for larger working ranges without encountering points where the signal quality is below the threshold.
Here, it is important to distinguish between working range and working distance: The former refers to the range over which a measurement can be carried out, whereas the latter refers to the distance between sensor head and mirror. The working range is therefore the difference between the lowest and highest working distance encountered during a measurement. The conclusion is that in order to measure over large working ranges, there is a lower limit of the working distance that has to be respected. How to find this limit and choose the ideal working distance for your experiment will be explained in the following. Please note that working range and distance depend on the type of the sensor head. This page concerns only the small collimating sensor head PS-SH-C01 with a length of 13 mm and a diameter of 4 mm.
Determine Working Distance
Figure 2 shows the method how to choose the ideal working distance for a given desired working range, i.e. the maximum expected travel of the object mirror with respect to the sensor head during a measurement. In figure 2, this range is chosen to be 200 mm, meaning that during the experiment we expect a travel of 200 mm . The minimum distance, i.e. the lower bound of the working range, has to be within the green marked segment to allow such a measurement. Following the red arrow, one finds that the lower limit for the minimum distance is 71 mm.The upper limit of WDmin, shown by the blue arrow, is only confined by the maximum working distance of the interferometer of 650 mm and is therefore 450 mm in this case. This means that the distance between sensor head and mirror during the measurement should be from 71 mm to 271 mm to ensure an optimal signal-to-noise ratio. If the constraints of an experiment dictate longer distances, the farthest possible location of the working range would be from 450 mm to 650 mm distance.
Infer Working Range
The red arrows in figure 3 show the opposite case. For a given minimum working distance on the y-axis, the maximum working range can be found on the x-axis. In the case shown by the red arrow, the maximum working range for a lower distance limit of 100 mm is found to be 320 mm , which means that reliable measurements can take place, if the object mirror is located between 100 mm and 420 mm away from the sensor head. This can be important when the distance between sensor head and mirror is somehow restricted by the measurement setup.
The above results are only valid for the small collimating sensor head PS-SH-C01. Larger sensor heads allowing for even greater working distances are currently under development and will be available shortly.