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Model 5501A
Theory of Operation
SECTION Ill
THEORY OF OPERATION
3-1.
INTRODUCTION
3-2. This section provides the theory of operation for the laser head. The first part presents general laser
theory a s it applies to the laser head. Subsequent paragraphs described detailed laser head operation;
functional analysis of the laser head is included.
3-3.
GENERAL DESCRIPTION
3-4. The laser head transmits a coherent light beam (all light waves are in phase) that
i s
used by the laser
transducer system to generate displacement measurement signals. In addition to this beam, the laser head
generates an electrical reference (REF) signal, and accepts and produces interface and diagnostic signals for
accessory equipment. The laser head accepts +15 Vdc and -15 Vdcoperating power from an external source
and distributes fused +I5 Vdc and -15 Vdc to other units of the transducer system.
3-5. The laser head basically consists of a laser tube assembly, regulator circuits that ensure optimum laser
operation, and diagnostic circuits.
3-6. The laser tube contains a Helium-Neon gas that is excited when high voltage is supplied. A laser
current control circuit maintains the appropriate laser tube current by monitoring cathode current and
adjusting the high voltage accordingly.
3-7. The laser tube consists of the anode, cathode, mirrors, a spring, and a piezoelectrictransducer.These
elements are enclosed in the Helium-Neon environment. As a result of the exictation, light energy in the
form of photons are spontaneously emitted by the excited Neon atoms. These photons, traveling
approximately at the speed of light, are reflected by the mirrors and collide with Neon atoms that are in a
metastable state. This collision results in the stimulated emission of several photons by the Neon atoms. This
event occurs repeatedly and is responsible for the laser phenomenon; Light Amplification by Stimulated
Emission of Radiation. Further photon collisions cause increased coherent emission. These chain reactions,
ultimately create an in-phase, or coherent light energy level which is sufficient to generate a beam through
the laser tube aperture.
3-8. The laser frequency is determined by the transition between energy levels of the Neon atoms. The
distance between mirrors establishes a cavity length which isadjusted tosupport longitudinal oscillationsat
a
wavelength of 6328 Angstroms (5 x 1014 Hz). This wavelength lies in the red region of the visible light
spectrum.
3-9.
A
small amount of resonant cavity length tuning is provided by the piezoelectric transducer (PZT)
which is in front of the rear mirror. A spring behind the mirror forces it against the PZT. The PZT has the
property of expanding to a thickness which is proportional to the amount of positive dc voltage applied
through a stem connection at the rear of the tube. The expanding PZT pushes the mirror to the rear of the
tube, thereby creating a longer resonant cavity. The longer cavity sustains oscillations at a slightly lower
frequency. Therefore, the laser tube responds to a more positive PZT voltage by tuning to a slightly lower
frequency. Conversely, the tube responds to a less positive PZT voltage by tuning to a higher laser
frequency. This PZT control potential ranges from +270V to +1800V.
3-10. A magnet that surrounds the laser tube causes Zeeman splitting of itsfrequencysymmetrically about
f , , the normal laser center frequency. This results in two circularly polarized frequency components existing
in the same beam. One component i s left-hand circularly polarized (LHCP) and is approximately 1 MHz from
the center operating frequency of the tube (f,). The other beam frequency component is right-hand
circularly polarized (RHCP) and is approximately 1 MHz from f , ,
in the other direction.
3-11. The laser beam, containing the two circularly polarized frequency components (f, and f,), passes
through a A/4 plate (A =wavelength). This causes the f, and f,components to become linearly polarized and
mutually perpendicular, or orthogonal. These frequency components then pass through a A/2 plate which is
factory-adjusted to compensate for the imperfect orthogonal positioning of the f, and f, signals.
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