55
XL80 laser system
Figure 4
Linear measurement optical set-up�
With the optics configured as shown above, the sign convention is as follows:
•
For positive movement (measurement retroreflector moving away from
XL laser) A leads B�
•
For negative movement (measurement retroreflector moving toward
XL laser) B leads A�
1
XL laser
2
Reference retroreflector
3
Linear beam splitter
4
Measurement retroreflector
Direction sign convention
4
2
1
Update rate
The quadrature output update rate is 20 MHz�
Accuracy
The transitions of the quadrature signals are accurate to within ±10 nm at low
velocities� However, there is a very small propagation delay (D) between a
change in the optic position and in the quadrature output� The true accuracy
of the transition (ignoring air refraction errors, see below) is given by:
Accuracy = ±(10 + Dv) nm
where v = velocity in m/s
D = 600 nsec
Delays due to any customer interface should be added to D to obtain an
estimate of transition accuracy in a specific application.
Wavelength environmental compensation
The quadrature outputs are not compensated for changes in the refractive
index of air� The value will be subject to variation dependent on environmental
conditions� See
3
