Distance And Velocity Measurements - HP 5528A User Manual

Model 5528A
Appendix A; Principles o f Operation
A-6
Distance and Velocity Measurements
The F1 and F2 beams that leave Ihe laser Head are aimed al an Inl erferom ter which split s Fl and
F2 via a polarizing beilm spl ittcr. (See Figure 1\-2). Component Fl becomes the fi xed-distil nce
path , and F2 is sent 10 a larget which reflects it back to the Interferometer. Relative motion
between the Interferometer and the remote Retroreflectorcauses a Doppler sh ift in the returned
frequency. There fore, th e Measurement Receiver photodetector in the l aser Head seeS;1 fringe
difference given by fl - F2
±
aF2.
The F1 - F2
±
llF2 sig nal that is returned from th e external Interferome ter is compared in the
Measurement Di sp lay unit to the
Fl - F2
Reference sig na l. The difference,
dF2,
is then related
electronically to veloci ty and then to di stance. This informati on is finally man ipulatcd as
described above und er "G ENERAL SYSTEM PR INCIPLES" to produce th e correct ou tput on the
d isp lay.
Proper separati on between o ut goin g and incoming laser beams is provi ded by each Retro-
ref lector.
REFEREN CE
RETROAEFlECTOR
MEASUREMENT
/'-
AETROREFlECTOA
/
h
"-
I,
I,
V
I,
"-
FROM LASER
1 /
>
TO
I, f2 ± ~ 12
' 2 i ~ 12
PHOTO DETE CTOR
~
LINEAR INTERFER OME:'
FIgure A-2. D,sfance and Velocity Measuremenls
Angular Measurements
The set-up for making an Angular measurement is shown in Figure 1\-3.
The Angula r Opt ics creil te two parallel beam paths between the Angular Interferometer and the
Angu lar Reflector ; one pa th is frequency", the second path is frequency f2. The distance
between the two paths is preci sely k nown, since the
two
Retrorefieclors are precisely positioned
within lhe Angu lar Reflector. Initially, the angle between the Angular Interferometer and the
Angular Reflector is assumed to be "0", and the two paths between them have some relative
length .
If
either th e Angular Int erferometer or the Angular Reflector is turned, the relative
length s o f Ih e two path s will change: th is change wi ll cause a Doppler-shilled frequ ency change
in the beam tha t is ret urned from the Angular Interferomete r to the Laser Head. The Laser
Measurement Syste m's Electro ni cs hardwa re wi ll ca lcu late and display the angular change, using
the new frequency co unt data and th e spacing between th e opt ica l path s.