Carbon dioxide laser frequency stability data are considered for a receiver design that relates to maximum Doppler frequency and its rate of change. Results show that an adequate margin exists in terms of data acquisition, Doppler tracking, and bit error rate as they relate to laser stability and transmitter power

Johnson, E. H., Jr.

English

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THE CO, LASER FREQUENCY STABILITY MEASUREMENTS 
Edward H. Johnson, Jr. 
In the previous paper, one of the major problems with a laser receiver at synchronous altitude 
was discussed, that of Doppler shift, which can be as much as plus or minus 700 megahertz. 
Associated with that is also a maximum rate of Doppler change, which approaches a mega- 
hertz per second. I might add that the 10.6-micron laser frequency is 28 terahertz. 
In addition to the problems imposed on receiver design that relate to maximum Doppler 
frequency and its rate of change, the stability of the laser is also of primary concern. Up to 
this date, we have had very little data on actual stability of the lasers in terms of phase noise, 
and in order to examine the problem and use it as an input into tracking receiver design, we 
implemented an experiment in the laboratory to measure and evaluate these stability effects. 
Figure 1 indicates the objectives of the experiment. Primarily, we want to obtain experi- 
mental data on laser stability which affects the phase-lock loop tracking bandwidth, thus 
imposing minimum-bandwidth considerations. Another consideration is Doppler tracking 
performance. We would like to make phase-lock loop bandwidths wide for acquisition and 
tracking purposes; but conversely, we want to make them as narrow as stability will permit 
for best signal-to-noise ratios. The third effect is on a bit error rate. Our system is a coherent 
on-off keying system, and for that, the probability of making an error is, in general, one- 
half the complementary error function of the square root of the signal to four times the 
EXPERIMENT OBJECTIVES 
1. OBTAIN EXPERIMENTAL DATA ON LASER STABILITY 
FOR DESIGN OF 10.6-MICROMETER RECEIVER 
a)  PHASE-LOCK LOOP TRACKING BANDWIDTH 
b) DOPPLER TRACKING PERFORMANCE 
c) EFFECT ON BIT ERROR RATE 
d) FREQUENCY ACQUISITION 
2. DETERMINE SOURCE OF INSTABILITIES OBSERVED 
Figure 1. Quantitative COz stability data required for receiver terminal 
design. C02 laser phase noise data not available. 
191 
https://ntrs.nasa.gov/search.jsp?R=19730019129 2020-03-17T09:02:18+00:00Z
noise. However, if there is an inability to reconstruct the carrier, you get a cosine square 
of theta-rms, which is caused by oscillator instability. Another concern is the problem of 
frequency acquisition. If it is possible to open your loop bandwidth wide, you can mini- 
mize frequency acquisition problems. 
The second major purpose of the experiment was to actually look for instabilities in laser 
power supplies, acoustical vibrations, and other sources of instabilities. 
The actual experiment configuration is shown in Figure 2. We used two CO, lasers, shown 
on the left, and with FTZ tuners, we were able to pull the frequencies of each laser until 
we had a nominal 20-megahertz beat frequency between the two. 
LASER LASER 
# I  
LASER 
#2 LASER 
EXPERIMENT CONFIGURATION 
RMS PHASE 
NOISE VOLTMETER 
LOOP FILTER 
MIXER 
I - 
7712 PHASE 
SHIFT 
=20 MHz - P 
1 
ANALYZER 
MIXER 
COMPUTING SPECTRAL DENSITY Fi-l ~1 OF AMPLITUDE FLUCTUATIONS 
SPECTRAL DENSITY 
I OF AMPLITUDE FLUCTUATIONS VARIANCE OF 
FRACTIONAL FREQUENCY 
FLUCTUATIONS 
Figure 2, Experiment implemented to obtain desired data: spectral density and variance measurement. 
This signal emerged from the optical mixer, and was passed to a phase-lock loop tracking 
receiver. At the output of this receiver, we could measure rms phase noise within a loop 
bandwidth, and with a spectrum analyzer, we could get a spectral density measurement of 
the frequency fluctuations. 
192 
A third measurement we made was down through a quadrature loop. This would be a 
coherent demodulation of amplitude fluctuations, and we can get the corresponding 
spectral density. 
A fourth measurement we made was in the time domain, using a computing counter. We 
actually measured and plotted the variance of the fractional frequency fluctuation. This is 
a typical signal versus tau plot that you see in the time domain. 
The result of what we’re after is shown in Figure 3, in which we used the data resulting from 
the experiment. This plot shows the phase-lock loop tracking error, or your inability to 
reconstruct the carrier versus the tracking loop bandwidth. The ordinate here is the error 
in degrees rms versus the phase-lock loop bandwidth in kilohertz. The line running from top 
left to bottom right is the result of the data from the stability measurements. Basically, it 
implies that there is a minimum loop bandwidth that you can use. 
The horizontal lines - nominally 30 degrees rms phase error - are values where the phase- 
lock loop will skip cycles and drop out of lock. At 20 degrees’ error, some degradation of 
the data signal-to-noise ratio will occur. These are thus maximum values that we cannot 
exceed. 
PHASE-LOCK LOOP TRACKING ERROR VERSUS LOOP BANDWIDTH 
(Low Earth Orbit to Synchronous at 10.6 pm) 
v) 
E, m 
I 
n 
0 n 
w 
a 
c.9 
Y 
0 
n 
I- 
z 
a 
1 
P =  
--x-- /A<* .E. n -_I LOOP SKIPS CYCLES 
i 1 u.uu I v v n  I I 
0.1 I I I 1 1 1 1 1  I I I 1 1 1 1 1  I I I I I  1 1 1 1  I I I I I I I I I  
1 oo 1 0’ 1 o2 1 o3 1 o4 
PHASE-LOCK LOOP BANDWIDTH (kHz) 
Figure 3. Data used with other receiver parameters. Adequate margin exists in 
terms of acquisition, Doppler tracking, and bit error rate as they relate to laser 
stability. 
193 
The three sloping lines on the right are for various transmitter powers on an earth orbit 
satellite up to synchronous altitude. As you can see, for a signal level of, say, a milliwatt 
in the carrier, there is an optimum loop bandwidth at which we should operate to minimize 
the phase error. 
Corresponding to this bandwidth, there is a maximum Doppler rate associated with it. The 
basic result is that we have measured the data on the lasers, and there is a margin, as you can 
see, for these various transmitter powers. 
194 


The CO2 laser frequency stability measurements

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