HP 5528A User Manual

Model 5528A
AppendiJ( A: Principles of Operation
A-4
Ea ch beam from th e Straightne ss Interf eromc ter Is directed to one mirror of the Straightness
Reflector. The Straightness Reflect or consists of a pair of front-su rfa ce pl ane mirror s, rigidly
mounted such that they reflect each beam back along it s incoming path to the Int erferometer.
PRINCIPLES OF MEASUREMENTS
General System Principles
A Helium -Neo n gas la ser in the laser Head produces a cohe rent li ght beam (s ee Figure A-1 ). A
cy lindrical perma n en t magnet around the cavi ty causes the la ser to osci ll ate at two slightly
different fr eq uencies. This is Zee man spH nin g. The two compo n en ts, F1 and F2 1" frequen cy 1"
and "f r equency 2") are distinguishable by their opposite ci r cu lar polari zation s.
From the la ser tub e, the beam co ntaining both the Fl and the F2 frequencies passes through
Optical g la ss quarter -wave and half-wave pl at es which change the ci rcular po larizations of F1 and
F2 to lin ear perpendicular polarizations, one ve rt ica l and the other horizontal. The lin early
polari ze d b ea m is eJ(panded in a co llimating telescope, after which most of the beam Is
transmilled thr ough a 45-degree beam splin er and out of the la ser Head.
From the
45 - degre e beam splitter, a porlio n of the beam 15 sa mpled. Most of this sample is fed
into
the Referen ce Re ce iver to detec t the difference In fr equen cy between the beam's two
op ti cal co mponent s ( 11 and f 2). This difference will be between 1 .5 and 2.0 MHz, and is
r
eprese nted in Fi g ur e A-1 as " fl - f2". The r emainder of the u mpled beam con tr ols the tuning of
the lOI sel, to k ee p the la ser's ou tput frequency co nstant.
During a measurement,
the la ser beam is directed through Optics in the measur emcn t path and
then return ed to the La ser Head's Measurement Re ceiver. This receiver will detect part of the
returning beam that wi ll be (1 - (2 plus a Doppler-shifted frequency component, af, if there was
any relative motion between the Optics In the measurement path.
The t wo fre quen cies do n ot inte rf ere to form fringes until the beam reaches the demodu lating
po lariz
er mounted in front of the photodete ctor be cause th ei r polarizations arc orthogo nal to
each othcr.
Thc
signals from each of the la ser Head's r e<:e i vers ( Ref erence Rece iv er and Measurement Recei ve r)
are count ed by the Measurement Display in two separate pulse counter s, one for the Reference signal
and
one for the Measuremcnt signal. Th ese co unters store absolute count s, although the frequen cy
determines the rate at which thc cou nt s are accumulated. ( See Figure A- l )
The Measurement
Di splay unit then -
a.
Continously subtracts the Reference Counter's contents fr om the Meas ur emc nt
Counter's co ntents, -
(The
co unlcrs' co ntent s, and the difference va lue, are numbers of wavelengths
counted.)
b. Pe rf o rm s the necessary electronic functions to make the particular mcasurement
calculatio n s, -
c. Modifies the
results of " b" with keyboard input data ( va lu es entered using SET UP keys) and
compe nsation inform at ion from the Air Se nsor and Material Se nsor(s) ( if used ), and
then -
d. Displays the comput ed measur emen t on the front-panel display.

HP 5528A Specifications

General

Brand	HP
Model	5528A
Category	Measuring Instruments
Language	English