The CIRS Fourier transform spectrometer aboard the NASA/ESA/ASI Cassini orbiter has been acquiring spectra of the Saturnian system since 2004. The CIRS reference interferometer employs a laser diode to trigger the interferogram sampling. Although the control of laser diode drive current and operating temperature are stringent enough to restrict laser wavelength variation to a small fraction of CIRS finest resolution element, the CIRS instrument does need to be restarted every year or two, at which time it may start in a new laser mode. By monitoring the Mylar absorption features in uncalibrated spectra due to the beam splitter Mylar substrate, it can be shown that these jumps are to adjacent modes and that most of the eight-year operation so far is restricted to three adjacent modes. For a given mode, the wavelength stability appears consistent with the stability of the laser diode drive curren.t and operating temperature

Brasunas, John C.

NASA Technical Reports Server

June 29, 2012 
Laser Mode Behavior of the Cassini CIRS Fourier Transform Spectrometer at Saturn 
John C. Brasunas 
NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA 
HIGHLIGHTS 
• Cassini CIRS Fourier transfrom spectrometer in orbit about Saturn 
• Wavelength calibration via temperature-stabilized laser diode 
• Laser mode may change after instrument tum-off/turn-on cycle 
• Characterize mode change via Mylar features due to beam splitter substrate 
ABSTRACT 
The CIRS Fourier transform spectrometer aboard the NASAIESAIASI Cassini orbiter has been acquiring 
spectra of the Saturnian system since 2004. The CIRS reference interferometer employs a laser diode to trigger the 
interferogram sampling. Although the control of laser diode drive current and operating temperature are stringent 
enough to restrict laser wavelength variation to a small fraction of CIRS finest resoluton element, the CIRS 
instrument does need to be restarted every year or two, at which time it may start in a new laser mode. By 
monitoring the Mylar absoption features in uncalibrated spectra due to the beam splitter Mylar substrate, it can be 
shown that these jumps are to adjacent modes and that most of the eight-year opeation so far is restricted to three 
adjacent modes. For a given mode, the wavelengh stability appears consistent with the stability of the laser diode 
drive curren.t and operating temperature. 
Keywords: 
F ourler transform spectrometer 
Wavelength calibration 
Laser mode 
Mylar 
john.c.brasunas@nasa.gov 
+ I 301.286.3488 
+1 301.286.1683 (fax) 
https://ntrs.nasa.gov/search.jsp?R=20120017942 2019-08-30T23:29:36+00:00Z
1. Introdnction 
The NASA-ESA-ASI Cassini-Huygens space mission was launched in 1997, arriving at the 
Satumian system in 2004. The Cassini orbiter continues to operate and return data to Earth. Among the 
remote-sensing instruments aboard Cassini isCIRS [1], a Fourier transform spectrometer (FTS) with a 
50.8 cm input telescope and a dual FTS configuration with a common moving-mirror mechanism [Fig. 
1]. The far-infrared (FIR) FTS covers the 10-600 cm-! spectral region with the FPl (focal plane 1) 
thermopile detectors at 170K, isothermal with most of the optics. The mid-infrared (MIR) FTS covers 
the 600-1100 and 1100-1400 cm-! regions with two HgCdTe focal planes (FP3 and FP4). FP3 and FP4 
are cooled to near 80K with a passive radiator. Most electronics are housed separately at 300K on the 
Cassini spacecraft. The spectral resolution is selectable by command, ranging from 0.5 to 15.5 cm-!, 
corresponding to scan times of 50 and 2 s, respectively (round-trip). 
Telescope and 
interferometer optics 
@ 170K 
MIR FTS 
r---------- .. 
• I 
• I 
I I 
: I ! FP3/4 @! 
• 8(}K : 
: I 1-----______ ' 
Cassini CIRS FTS 
Mechanism 
reference interferometer 
detector 
Fig. 1 elRS optical-mechanical layout (not to scale) 
most electronics 
neai300K 
50 em primary 
FIR FTS 
FP1 @ 
170 K 
2. Steps taken to stabilize CIRS wavelength scale 
To make data calibration practical, it is necessary to make the sampling of the FTS raw data, the 
interferogram, as repeatable as possible from scan to scan. As is common with the FTS, eIRS includes a 
so-called reference interferometer that utilizes the MIR moving mirror. The reference interferometer 
includes a light-emitting diode, to signal the start of the forward scan, and a laser to trigger the sampling 
ofFP1,3 and 4. As the eIRS laser is a single-mode A1GaAs laser diode SDL 5601 [2], its wavelength 
(nominally 783 nm at 170K) and thus the eIRS sampling comb stability is limited by the stability of the 
temperature of the laser diode [Fig. 2] and by the stability of the laser drive current [Fig. 3] 
· . . 
170 · , . M • "," " • "" • " ~ •• " " • ~ " • "," ~ • " •• " ••• _ •• " •• ", ••• " • " " •• " •• " " • " 
· . . , , . 
· . . 
· . 
'69.95 ··_"·--'1:············:-········ .... j ..... ! .................. . 
:1· I ::: . ,:. : : 
:: i !; : '!: ',!': ~ : : I 
-, I :! : II ': ! ,. h ' I • , 
...... ._.. ._ .. _.1111;--1' 1,'·,· ~~iW· .•.. II!. ,,1··+ ._._ .........•.. i 'ii 
: l: ':1 : j. I ;!" .::!,. i : 1; ! I 
: :: 1 :: " ! : j: . 
: f' :: ,!" il ': 
...... .. .•...... . .!!lkI Uh • !i 
i:!; );1." ji~! i .~.Diil~~ I 
169.8 .. .. .... .'f:·P, .~if i 
!. , 
169.75 . , • N •• 'N •• "" •• " • ~ " •• " • ~ .'. ~ • " •• " •• " ~ ., " ••• ' •• " " • ~ " •• " •• "" 
• ,. I 
••• ~ •• ft " •• " •• " _; •• " •• " •• " ••• " • " " _:_ " •• " " •• " • _ •• _ • ":" " 
1.05e+09 1.le+09 1.15e+09 1.2e+09 1.25e+09 1.3e+09 1.35e+09 
Fig. 2 Laser temperature (K) VS. UTe time (s) from 2004 to the present (50 million S N 18 months) 
The long-term temperature drift over ~ 8 years appears to be about 0.01 to 0.02K. With a typical 
between-mode-hops laser coefficient of ~ 0.08 nmlK, the long-term laser wave1engh drift should not 
exceed ~ 0.0016 nm, with a shorter term variability ofO.1K corresponding to 0.008 nm. Give.n a 
characteristic laser driver current effect (a change of20 rnA corresponding to ~Ph temperature change), 
the 0.016 rnA quantization level of the laser current corresponds to ~ 0.0002 nm. These shifts and the 
typical laser mode spacing are compared with the eIRS spectral resolution in Table 1. 
22.42 
22.4 
22.38 
--- --- J~ ---- ·--.. ----:-------.;---------7'--
22.36 
22.34 
22.32 -. 
1.05e+09 1.le+09 1.15e+09 1.2e+09 1.25e+09 1.3e+09 1.35e+09 
Fig. 3 Laser drive current (mV) UTe (5) over course of the missicln 
Table 1 laser diode characteristics compared with Ce~p spectral resolution 
Environmental Variation Limiting factor Laser line Fractional 
Factor (nm) change 
Thermal- 2 minutes O.OIK thermal inertia 0.0008 11100 
Thermal - 24 hours O.IK active control 0.008 11105 
Thermal- year-to-year ~ 0_02K aging? 0.0016 21106 
trend over 8 years 
Bias current stability 0.016 rnA quantization 0.0002 1I(4xlOO) 
Mode spacing E~m = 1) integer spacing cavity length ~ 0.2 114000 
FPl spectral resolution 0.5 cm- l scan length ----- 111200 
FP4 spectral resolution 0.5 cm-! scan length ----- 112800 
3. Evidence of changing laser mode over the course of the Cassini mission 
Table I demonstrates that the CIRS environmental stability is good enough to keep the laser 
wavelength change to a small fraction of the highest relative spectral resolution at the high ends of the 
various CIRS focal planes. It is also important to maintain laser diode operation within a single mode. 
For the CIRS flight model lasers at the 170K operating temperature, the lasers remain in a single mode 
operation over a wide range of drive currents. While the moving mirror is scanning forward, the 
reference laser fringes trigger the AID sampling circuit at ~ 1KHz. A rectified and low-pass filtered 
version of the laser signal (sampled every second) is stored in the housekeeping data, keeping a record of 
the envelope of the interferogram signal from the reference interferometer. The envelope signal at 3 
seconds after start of forward scan is shown in Fig. 4. 
II 
5.5 "0"' 
5 
4.5 
1.05e+09 1.le+09 
I 
I 
I 
~I~ 
i 
I 
1.15e+09 1.2e+09 1.25e+09 
• ~ 0< " 
, •• ~ 
. ! . ~ I ... :. W~W .• ~. ~ 
, " 
" , 
." " 
1.3e+09 
Fig. 4 Reference laser envelope signal (V) at .35 after start of forward scan, v~. UTe IS) over course of mission. 
N.PReHl~ 
The jumps in Fig. 4 are apparently mode hops. These mode hops correspond to times when the 
CIRS instrument needed to be turned off or switched to the sleep mode. When the laser is turned back on, 
single-mode operating is acquired shortly, but without any guarantee that the mode after turn-on will be 
the same as the mode before turn-off. From the above figure, it appears intervals 1 and 4 are in one mode, 
intervals 2, 3, and 3p(rime) are in another. It is not known if the slope ,within modes corresponds to an 
actual drift in wavelength, nor it is understood why interval 5 has a different slope at an atypical spacing 
relative to the other modes. Nor is it clear what the relationship is between laser envelope signal and laser 
wavelength, other than to indicate when the laser mode has changed. 
4. Method to quantify laser modes: spacing, and within-mode wavelength stability 
CIRS has an on-board calibration target, but it is a broad-band thermal target for intensity 
calibration. There is no on-board wavelength calibration target, and the reference laser itself is not locked 
to any atomic or molecular feature. It is possible to calibrate the wavelength by comparing observed and 
theoretical spectra of atmospheric features on Saturn or Titan. However this is time-consuming and 
requires careful attention to detail in a multi-layer atmospheric model with a priori unknown distributions 
of temperature and chemical abundances. Are there any useful spectral signals in the calibration targets 
themselves? The answer appears to be yes for FP1, but not for FP3 or FP4. The FIR FTS (Fig. 1) is a 
polarizing-type FTS; the beam-splitter is actually a wire-grid on a Mylar substrate. Fig. 5 shows the 
average of ~ 5,000 un-calibrated FP1 spectra taken at the highest spectral resolution, with Mylar features 
marked as A, Band C [3]. 
I 
I I 
20000 JJJ~JJJJJJJ~JJJJJJJ~JJJJJJJJfJJJJJJJJ
I 
15000 I I ------ - -,- - - - - - - -,- - - - - - -
I I 
I I 
I I 
I 
I I 
10000 interference - -1- - - - - - - -1- - - - - - __ I 
5000 - - - - - - - - - - - - - - - - - - - - - - - - - - -
I 
------.--- ____ • _______ ._. _____ ~JJJJJJJ ... --------I------
I I I I I I 
100 200 300 400 500 600 
Fig. 5 FP1, showing Mylar and interference features; un-calibrated spectrum vs. em·1 
I 
I 
I 
I 
I 
I 
700 
The idea is to use the Mylar features to study the laser mode hops; it is not expected that the 
Mylar features would exhibit sudden changes in position. There is also an interference feature near 200 
cm-1, corresponding to 8 Hz. This features in due to digital communications between CIRS and the 
Cassini spacecraft, which add coherently scan-to-scan because the repeat cycle of the CIRS scans is 
locked to a multiple of the spacecraft clock (especially near the beginning of the mission). The 
interference fundamental has harmonics near 380 cm- l and 570 cm- l , making the use of Mylar feature A 
problematic. Fig. 6 shows the Mylar features in greater detail. 
The central idea then of this paper is to create averages of ~ 4,000 to 5,000 un-calibrated FPl 
spectra taken at the highest spectral resolution, for each of the numbered, ~ month-long-periods shown in 
Fig. 4. The un-calibrated spectra are interpolated to a resolution of /:lcr ~ 0.0326 cm- l . Taking any two 
periods, with corresponding spectral averages spect1(cr) and spect2(cr), compute the root-mean square 
difference diffn: 
diffn = .E ((spectl ((J) - spect2((J + ml1(J)) **2/ (spect1 M * spect2((J + ml1(J ))) 
where the spectra from the two periods are offset an integral number m (+/- 32) of resolution elements 
/:lcr, and the limits of summation correspond to the two windows for Mylar features Band C. 
18000 
__ I __________ l __________ L _________ J __________ L __________ L 
I I I I I 
I I 
I 
I I 
I I I I I 
--,---------- ----------r---------,----------r----------r 16000 
I I I I I I 
I I I I I 
I I I 
I I I 
I I I 
14000 J~JJJJJJJJJJ~JJ JJJ~JJJJJJJJJ~JJJJJJJJJJ~JJJJJJJJJJ~ I I I I 
I I 
I I 
I I 
12000 
_~ __________ l ___ _ 
I I 
I 
lDOQO I I J~JJJJJJJJJJIJJJJJ
I I I I 
sooo 
: : ~W I:l 
--:- ---------1--J~ -J~ -----~J -------
I I I 
I I 
I I 
I I I 
I I I I I _~__________ l __________ L _________ ~ __________ L 
I I I I 6000 
I I 
I , 
300 ]50 400 450 soo 550 
Fig. 6 Zoom in on Mylar features in FPl, un-calibrated spectrum vs. cm-l • 
Mylar feature B: 
Mylar feature C: 
420 to 450 cm- l 
500 to 515 cm- l . 
A typical plot of diffn for window C is shown in Fig. 7 (the offsets 0, +/-32 have been re-mapped to 
1:65). Feature C has a stronger sensitivity to changes in wavelength, with a 0.95 change in diffn for this 
range of offsets, versus ~ 0.35 change for feature B. An offset of one unit of shift corresponds to a 
wavelength change of ~ 1113,300 for Band 1115,600 for C. A parabola is fit to the minimum to define 
the location of the minimum with sub-integral resolution. 
l.aIJJJJJJJJJJJJJJJJJJJJJJJJJJJJJJJ~JJJJJJJJJJJJJJJJJJI 
I 
I I I I I I 
0.9 
______ l _______ ~ _______ J _______ ~________ L _______ L __ 
I I I I I 
0.8 I I I I I I ----r-------T-------,-------,--------r-------r 
I 
JJJJJJJ~JJJJJJJ~JJJJJJJ~JJJJJJJ~JJJJJJJJ~JJJJJJJ~JJJJJ JJ
I I I I I I 
I I 
I I I I .1 I 
Q.6 _l _______ J _______ J _______ J ________ L ______ L ______ _ I I I I I I 
I 
I 
I I I I I 
JJJJJJJfJJJJJJJlJJJJJJJIJJJJJJJ~JJJJJJJJrJJJ ---r-------D.S 
I I 
OA JJJJJJJ~JJJJJJJ~JJJJJJJ~JJJJJJJ4JJJJJJJJ~J JJJJJ~JJJJJJJ
I I I I I I 
I 
I 
0.3 
_______ L ______ _ 
I I I I 
-------r------- JJJJJJJ~JJJJJJJJfJJJJ0.2 
I I I 
I 
I '- I I I 0.1 - - - - - - - T - - - - - - - 'T - - - - - -,- - - - - - - -,- - - - - - -r - - - - - - - r - - - - - --
I I 
l.l;JJJ~JJ~JJ~JJH_JJ~JJH_JJ~JJr_JJ~JJr_~~~r_~JJ~ 
10 20 40 50 60 70 
Fig. 7 diffn for Mylar feature C vs. shift ('33' corresponds to no shift) 
5. Results and conclusion 
The FPl data details for the six periods of Fig. 4 are given in Table 2. SCET (SpaceCraft Event 
Time) is another name give to spacecraft UTC time. RTI (real-time interrupt) is the trigger-signal 
supplied by the Cassini spacecraft at 8 Hz. The longest ClRS scans are ~ 50s or ~4MM RTI. ZPD (zero 
path difference) is the location of the FP 1 zpd relative to the reference interferometer start of scan signal. 
AMP is the interferogram amplitude at ZPD. ZPD and AMP are used to select either deep-space scans or 
scans with weak external targets, so as not to mask the Mylar feature. NPTS is the minimum number of 
interferogram samples available for long scans for each period. All theses FPl scans are for no co-
adding, and additional selection on a criterion NOISE=1 has been made to reject some known problems 
with the data. 
Table 2 FPl data details for various periods to be analyzed for changes in laser wavelength 
Period 1 2 3 3p 4 5 
Month(s) Late 2004 January June 2010 May 2010 January June-July 
2009 2011 2006 
SCET 1080777600 1230768000 1275350400 1272672000 1293840000 1149120000 
1101859200 1233446400 1277942400 1275350400 1296518400 1154390400 
RTI 400 401 401 401 401 401 
ZPD 89.4-89.48 89.37-89.45 89.38-89.46 89.38-89.46 89.39-89.47 89.36-89.42 
AMP -11000 -11000 -11000 -11000 -11000 -11000 
-10500 -10500 -10500 -10500 -10500 -10500 
NPTS 2755 2720 2718 2720 2719. 2720 
Temperature 169.795 169.79 169.78 169.78 169.78 169.79 
(K) 169.82 169.88 169.86 169.88 169.86 169.89 
Scans 4473 3949 2828 3575 5065 3692 
The estimates of laser shifts are given in Table 3, comparing pairs of periods to estimate either the 
uncertainty (adjacent periods), mode shifts, or wavelength trends within a mode. The interferograms are 
separately processed for each pair-wise comparison, trimming the interferograms in both periods to a 
common, maximum possible length. Un-calibrated spectra are produced, averaged over each period, and 
the minimum of diffn is estimated via parabolic interpolation on diffn. Recall that a minimum located at 
33 corresponds to no shift, and the unit of shift (a 34 instead of a 33) corresponds to a shift of 0.0326 
cm- I . The uncertainty in the shift estimate appears to be ~ 0.3 for the 500-515 window, and somewhat 
more for the 420-450 window. The mode shift seems to be ~ 2.8 (period 4 to 3), and 3.0 (period 3 to 5). 
Thus the modes appear to be equally spaced as they should be, and the implied wavelength change is ~ 
115000 to 114400, quite close to the nominal laser mode spacing as reported in Table 1. Thus these appear 
to be jumps between adjacent modes (ilm = 1). As for the within mode trends, Fig. 4 shows a 1 to 4 
change ~ 20% of the mode jump at 4 to 3. This would be a shift of 0.6 if Fig. 4 is assumed proportional 
to the laser wavelength. This is not seen in the 1 to 4 comparison for the 500-515 cm-I comparison, and is 
within the noise for the 420-450 window. Not seeing a within-mode laser trend at this level is consistent 
with the stability of instrument temperature and laser drive current, which would indicate a wavelength 
trend less than 1 % of the mode spacing over the course of eight years. 
Table 3 Estimates of laser wavelength changes between various pairs of time periods 
Comparison 3 vs. 3p 4 vs. 3 3 vs. 5 2 vs. 3 1 vs. 4 
Purpose uncertainty mode change mode change trend in mode trend in mode 
420-450 cm-! 
window 
Minimum location 32.48 35.8 35.45 32.45 32.4 
Minimum of diffn 0.0032 0.0035 0.032 0.014 0.028 
500-515 cm-! 
window 
Minimum location 33.3 35.8 36.6 33.13 32.9 
Minimum of diffn 0.0055 0.006 0.03 0.008 0.028 
Conclusion Uncertainty ~ Llm= 1 Llm= 1 less than Fig. 4 less than Fig . .4 
Llm= 0.1 implies implies 
In conclusion, the CIRS reference interferometer provides an indication of a change in laser 
mode after CIRS instrument restart. However the relation between laser wavelength and this indicator is 
not monotonic. By using Mylar absorption features originating in the beam splitter substrate, it is 
possible to identify spectral features present in all the intensity calibration scans of one of the infrared 
focal planes. From observed shifts of these Mylar features it can be shown these mode changes are jumps 
among three, equally spaced levels. It is also possible to confirm within-mode wavelength stability at a 
level consistent with the stability of the laser diode drive current and operating temperature over an eight-
year period. 
Acknowledgments 
The author acknowledges support from NASA's Cassini mission and from the Cassini CIRS 
calibration team. 
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Captions 
Fig. 1 elRS optical-mechanical layout (not to scale) 
Fig.2 Laser temperature (K) vs. UTe time (5) from 2004 to the present (50 million S N 18 months) 
Fig.3 Laser drive current (mV) Vs. UTe (s) over course of the mission 
Fig.4 Reference laser envelope signal (V) at 3s after start of forward scan, vs. UTe (5) over course of mission. 
Fig.5 FP1, showing Mylar and interference features; un-calibrated spectrum vs. em'l 
Fig.6 Zoom in on Mylar features in FP1, uncalibrated spectrum vs. em'l. 
Fig.7 diffn for Mylar feature e vs. shift ('33' corresponds to no shift) 


Laser Mode Behavior of the Cassini CIRS Fourier Transform Spectrometer at Saturn

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