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elaboration, from the output of photodiode PD1 a superheterodyne signal is extracted. This is a sinusoidal signal at 100 
kHz, whose phase depends on the distance AL to be measured through the following relation: 
AT 
AL 2 
Asynt e 
  
H(AL) = do + 
In Eq. (2), Agynt is the well known synthetic wavelength of the absolute interferometer, which is the result of the 
combination or the two optical wavelengths (Docchio et al, 1994). takes into account any optical path differences, 
(except for AL), arising from unwanted optical path changes due to mechanical vibrations and thermal changes of the 
optical set-up. In order to have a stable phase measurement for a given distance AL, it is necessary to build a phase 
reference signal containing the same unwanted fluctuations of p. In the case that the beams are perfectly aligned in 
the path between beam-splitters BS1 and PBSS5, it is possible to obtain a good phase reference signal from the PD1 
output. In fact, the phase difference between superheterodyne signals from PD1 and from PD2 depends only on AL. 
Actually, the elaboration electronic chain operates a demodulation of both signals from PD1 and PD2 and calculates 
the distance AL from the phase difference between the two demodulated signals. 
In dual wavelength interferometry the NAR of the displacement measurement is extended from the optical wavelength 
of the single-frequency interferometer up to the synthetic wavelength Asynt of the absolute interferometer; Asynt can be 
some order of magnitude greater than the optical one. The possibility to tune the NAR to the specific application allows 
us to adjust measurement parameters such as resolution and measuring range in order to optimize the system 
performance. Repeating distance measurements at different synthetic wavelengths, in turn obtained by tuning one 
source with respect to the other, yields a measurement whose NAR is given by the longest Asynt and whose resolution 
is given by the shortest one. The distance meter so far described has been combined to (i) a scanner unit and (ii) an 
autofocusing unit, to provide a non-contact, point-by-point gauging capability. 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
LOCKING 
ELECTRONICS ML 
2 
SL PD3 VICES hid 
s ELABORATION 
MZ. ELECTRONIC 
i A s PBS1 CHAIN 
V2 == BS2 — 
~ L1 A A 
» AO1 
. M Il r—-4-—-—-—-— —€— Tm cm 
BSP2 E 
I 3 | 
— | PD2 
L2 y m | 
Ao? ZN x75 Peg ^M | er 
. | «| MR 
[^ | |PBS5 — I N 
MS Past = BS1 | N | / 
| -— 
| 
  
  
  
PD1 7 
Fig. 3. Schematic layout of the absolute distance meter. ML: master laser; SL: slave laser; PBS1..5: polarizing beam 
splitters; BS1..2: non-polarizing beam splitters; M: mirrors; L1..2: partially reflecting plates; AO1: frequency shifter at 
80 MHz; AO2: frequency shifter at 80.1 MHz; P1..2: polarizers; M4, M2: retarding plates; MR: measuring arm 
retroreflector; RR: reference arm retroreflector; PD1: reference interferometer photodiode; PD2: measuring 
interferometer photodiode; PD3: beat-signal photodiode. 
| 
IAPRS, Vol. 30, Part 5W1, ISPRS Intercommission Workshop "From Pixels to Sequences", Zurich, March 22-24 1995