Document text
April 1969
Brief 69-10099
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program,
to encourage their commercial application. Copies are available to the public at 15 cents each
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151.
The primary function of the frequency control sys-
tem is to position the mirrors at either end of the laser
cavity so that the mirror separation is independent of
thermal and acoustical fluctuations. This condition is
achieved by splitting a small portion of the laser out-
put (10 microwatts) and directing it upon a photo-
detector (photodiode). The narrowband preamplifier
centered at 467 MHz then amplifies this signal by 26
dB, with an 8 dB noise figure. The noise figure of the
control loop is thus established, and the detected sig-
nal has sufficient power to be down-converted to 1
MHz in the mixer. The signal from the mixer is passed
onto the 1 MHz IF amplifier, where the bulk of the
system loop gain is realized. The bandpass character-
istics of the 400 kHz bandwidth IF amplifier are im-
portant in that any phase or amplitude distortion in
the detected signal will cause instabilities in the loop
compensation system. It was for this reason that a
“maximumly flat” characteristic was chosen.
The output of the IF amplifier is compared with a
1 MHz standard oscillator, which yields the phase
information as to which direction the piezoelectric
(com in ued over leal)
This document was prepared under the sponsorship of the National
Aeronautics and Space Administration. Neither the United States
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the
information contained in this document, or warrants that such use
will be free from privately owned rights.
transducer must move in order to compensate for any
fluctuations in cavity length. The amplitude of this
error signal determines the rate of compensation. An
integrator is used between the phase-sensitive detector
and the control elements, to convert the entire loop
to a first-order system with no net dc positional error.
In order for the system to function correctly, three
sinusoidal drive signals are required. The first signal
is the 467 MHz modulator drive signal. This signal
is supplied to a power amplifier which provides one to
three watts of power to the KDP (potassium dihydro-
gen phosphate) phase modulator used to couple laser
modes.
Since the change in phase of the 467 MHz beat
signal from the FM laser produces the discriminant
which operates the entire loop, the detected signal
must be heterodyned down to a lower frequency, while
preserving all the phase information. The heterodyning
is accomplished by phase locking the 468 MHz volt-
age-controlled local oscillator to the 467 MHz modu-
lator drive via a 1 MHz standard oscillator. As a
result, regardless of the precise frequency to which
the modulator drive is tuned, the local oscillator signal
is offset by 1 MHz, and the difference between these
is precisely fixed in phase with respect to the 1 MHz
standard. Therefore, as the optical cavity drifts ther-
mally or acoustically, the change in phase of the 467
MHz beat signal will be detected by the preamplifier
and IF amplifier, and will then form the discriminant
in the phase detector.
As a result of the phase information inherent in the
FM . discriminant, the error signal is always of the
correct sign. Thus the system acquires lock automati-
cally; and if lock should be lost, it is reestablished
without manual intervention.
One of the problems inherent in the stabilization of
lasers is that the amount of thermal expansion in the
laser cavity can be many optical half-wavelengths. The
piezoelectric transducer, however, can move through
only two or three half-wavelengths with maximum
voltage applied. In order to compensate for the slow
thermal drifts in the length of the laser cavity, a ther-
mal transducer was placed behind one of the laser
mirrors as shown in the diagram. This transducer is
simply an aluminum spool wound with heating wire.
When the voltage applied to the piezoelectric trans-
ducer begins to exceed the designated range, the trans-
ducer amplifier drives current into the heating element,
which in turn compensates for expansion and con-
traction of the laser cavity. This also has the effect of
keeping the integrator dc level constant at approxi-
mately 100 volts. The thermal transducer can move
the laser mirror through 80 half-wavelengths, and it
has a time constant of about 5 seconds. The water
flow through the base plate of the laser is adjusted to
reduce its thermal drift in order to keep the current
levels in the transducer at reasonable values.
Notes:
Documentation is available from:
Clearinghouse for Federal Scientific
and Technical Information
Springfield, Virginia 22151
Price $3.00
Reference: TSP69-10099
Patent status:
No patent action is contemplated by NASA.
Source: J. M. French, R. Targ,
J. M. Yarborough and L. E. Wilson
of Sylvania Electric Products, Inc.
under contract to
Marshall Space Flight Center
(MFS-14988)
Brief 69-10099
Category 02