HP 5501A Operating And Service Manual page 41

2.10 Measurement Optics
Each Laser Transducer axis must have an interferometer and a retroreflector. Machine design
considerations determine which type of interferometer is optimum. The choice of the inter-
ferometer for each axis usually specifies the retroreflector for that axis.
2.11 10702A Ll NEAR INTERFEROMETER AND 10703A RETROREFLECTOR (Figure 2-70). The
10702A Linear lnterferometer is the lowest cost unit and is used whenever possible for that
reason. The measurement retroreflector for this interferometer is the 10703A Retroreflector.
Displacement is measured between the interferometer and the cube corner. Either one or
both can move.
Figure 2-70. 70702A Linear Interferometer and 70703A Retroreflector
Normally, one is mounted on a moving part and the other is mounted on a fixed part and the
displacement between the two is measured. A diagram of this is shown in Figure 2-37. Note
that if this is a multiaxis installation each axis must be mechanically independent of the
other. In other words, if the Y-axis moves, the X-axis must be unaffected, so that the X-axis
retroreflector remains lined up with the X-axis laser beam.
If the linear interferometer must move, the 10702A, Option 001, must be used. For a detailed
explanation of why this option is required see Figure 2-77.
2.12 LINEAR INTERFEROMETER LASER BEAM PATH. The beam exiting from the laser
head is split at the surface of a polarizing beam-splitter, with one frequency reflected to the
reference cube corner mounted on the housing (Figure 2-72). The other frequency is trans-
mitted to the 10703A Retroreflector and returned parallel to, but displaced from, the outgoing
beam. Both frequencies are reflected back along a common axis to the photodetector in the
receiver, with one frequency including a Doppler frequency shift whenever the 10703A Retro-
reflector (or 107024 Linear Interferometer) moves.