Multi-chord fiber-coupled interferometer with a long coherence length laser

0.34 MB·English

by  Elizabeth C. Merritt

Multi-chord fiber-coupled interferometer with a long coherence length laser
Elizabeth C. Merritt,1 Alan G. Lynn,2 Mark A. Gilmore,1,2,a) and Scott C. Hsu3
1)Physics and Astronomy, University of New Mexico, Albuquerque, NM, 87131,
USA
2)Electrical and Computer Engineering, University of New Mexico, Albuquerque, NM, 87131,
USA
3)Physics Division, Los Alamos National Laboratory, Los Alamos, NM, 87545,
USA
(Dated: 25 January 2012)
2
This paper describes a 561 nm laser heterodyne interferometer that provides time-resolved measurements of
1
0 line-integratedplasma electron density within the range of 1015−1018 cm−2. Such plasmas are produced by
2 railguns on the Plasma Liner Experiment (PLX), which aims to produce µs-, cm-, and Mbar-scale plasmas
throughthe mergingofthirty plasmajets inasphericallyconvergentgeometry. Alongcoherencelength, 320
n
mW laser allows for a strong, sub-fringe phase-shift signal without the need for closely-matched probe and
a
J reference path lengths. Thus only one reference path is required for all eight probe paths, and an individual
4 probechordcanbealteredwithoutalteringthereferenceorotherprobepathlengths. Fiber-opticdecoupling
2 of the probe chord optics on the vacuum chamber from the rest of the system allows the probe paths to be
easily altered to focus on different spatial regions of the plasma. We demonstrate that sub-fringe resolution

] capability allows the interferometer to operate down to line-integrated densities of order 1015 cm−2.
h
p
-
m I. INTRODUCTION andtheprobepathlaunchandcollectionopticsallowsfor
longprobepathlengthsandforthelargeopticaltableto
s
a The study of merging supersonic plasma jets is rich in beplacedseveralmetersawayfromthe plasmachamber.
pl potentialphysics,aswellasservingasapotentialmethod The long coherence length of the laser (> 10 m) allows
. offormingimplodingplasmalinersrelevanttofundamen- path length variance between the probe and reference
s
c tal scientific studies of Mbar-scale, high-energy-density paths without the need to tailor the path length differ-
i (HED) plasmas and potentially magneto-inertial fusion ence to the periodicity of the laser coherence length.5
s
y (MIF).1 Determining plasma electron density, ne, dur- The long coherence length of the solid state laser, in
h ing jetpropagationandjet mergingis imperativeforun- conjunction with the fiber-optic setup, allows easy re-
p derstanding the underlying plasma dynamics. Electron configuration of the chord geometry. For example, the
[ densitydynamicscanyieldinformationonjetexpansion, chordpositionscanbeconfiguredtomeasurethedensity
3 potential shock structure formation, and kinetic energy profile of a single jet at different points along the axis
v changes across any merging boundaries. The Plasma of jet propagation, or for simultaneous measurements of
1 Liner Experiment2 aims to study the merging dynamics the plasma density before and after a merging boundary
2 of multiple plasma jets with the ultimate goal of creat- between two jets.
2
ingasphericalplasmalinerthroughthemergingofthirty
1
jetsinsphericallyconvergentgeometry. Tothisend,an8
.
1 II. EQUIPMENT
chord,visible, heterodyne,fiber-optic basedinterferome-
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ter has been designed and built for measuring ne during
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experimentsonone-jetpropagationandtwo-jetmerging, A. Optics
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: as well as the merging of thirty plasma jets. Individ-
v
ual plasma jets, which are generated and accelerated by The fiber optic components of the interferometer split
i
X railguns manufactured by HyperV Technologies,3,4 are the optics into three functionally decoupled stages: the
r expected to have initial density ne ≈1015−1017 cm−3 , initial establishment of the probe and reference chords,
a temperature1-5eV,andvelocity≈50km/s,correspond- the chord positioning optics on the chamber, and the
ing to Mach numbers of M ≈10−35. final probe/reference beam recombination. The initial
The visible wavelength of the interferometer was cho- establishment of the chords and the final recombination
sentoaccommodateadensityrangefromthatofanindi- takeplaceona3.05mx1.22mopticaltablelocatedata
vidualjet with peak line-integrateddensities ofR nedl = distance of approximately 6.1 m from the chamber. One
1016−1017 cm−2, throughatleastdensity doublingdur- of the primary advantages of fiber-coupling is the ability
ingthe mergingoftwojets,andfinallythroughR nedl = to place the bulk of the optics at a distance from the
1018 cm−2 corresponding to part way through liner con- chamberwithouttheneedtopreservedirectline-of-sight
vergence. Fiber-optic coupling between the laser source paths between the optics.
An Oxxius 561-300-COL-PP-LAS-01079, diode-
pumped solid-state, 320 mW, 561 nm laser6 produces
the initial beam. A 50 percent attenuation filter is
a)Electronicmail: [email protected] attachedto the aperture of the laser,bringing the trans-
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mittedlaserpowerdownto160mW.Abeamdivergence duces a collimated beam of ≈ 3 mm diameter over a
of1.2mrad,apathlengthof≈2mbeforebeingcoupled distance of 3 m. All eight probe beams pass through
into the fiber, and an initial beam diameter of 0.6 mm the chamber to an identical rectangular window/optical
renderscollimationopticsinthis stageunnecessary. The breadboard setup on a port on the opposite hemisphere
coherence length of the Oxxius laser is at least 10m as ofthe 2.74 mdiameter sphericalchamber. A set of eight
conservatively specified by the manufacturer,6 but the Thorlabsfiber-couplersaremountedonthesecondbread-
theoreticalcoherence length is much higher as estimated board. Turning mirrors on either or both breadboards
fromthelaserfrequencylinewidth6 of1MHzasfollows:7 route the probe beams from the collimators to the fiber-
c c couplers, where the beams are coupled into a second set
∆l =c∆t≈ = ≈100 m. (1) of 20 m single-mode fibers. The exact optics arrange-
∆ν 1 MHz
mentvariesdependingonthephysicsbeingstudied. Op-
Aslongaslengthdifferencesbetweenthereferenceand
tics may be positioned to measure the plasma density
probe paths are kept small compared to the laser coher-
throughthe centerofasinglejetateightdifferentpoints
ence length, fringe visibility will still be high.7,8 Bench
alongtheaxisofjetpropagation;thisarrangementisop-
testsofonechordoftheinterferometerallowedatleasta
timal for studying electron density evolution in the jet
2.4 m path length mismatch between the probe and ref-
as it propagates. The optics may also be positioned to
erencechordwithoutanyappreciablesignaldegradation.
measure the plasma density of the jet at one point along
This allows the use of one reference chord with respect
the axis of jet propagation but at different radial posi-
to all eight probe chords, since the probe chord length
tions within the jet; this arrangement is optimal for a
variationdue to the optics’placement on the chamber is
radialdensityprofile measurement. The othermajorad-
easily kept below 2.4 m.
vantageofthe fiber-optic systemis the decouplingofthe
Thorlabs 460HP single-mode fiber is used for all
optics that dictate chord placement in the plasma from
fiber paths before reference/probe beam recombination.
the rest of the system. This decoupling reduces the task
Single-modefiber,whilerequiringgreaterlaserpowerdue
of altering chord arrangement to the placement of ap-
to poorer coupling into the small fiber core, bypasses
proximately 32 components (four per chord) instead of
any signal degradation due to the interference effects
the realignmentofall110opticalcomponentsin the sys-
from uneven wave-packet spreading and delay in differ-
tem.
enttransverselasingmodesinmulti-modefiber.8,9 Thus,
Allprobeandreferencefibersareroutedtotherecom-
single-modefiberpreservestheinterferometer’sabilityto
bination optics. Each fiber terminates at an adjustable
function with chord path length differences. Thorlabs
focus collimator, where they are again collimated into
BFH48-400multi-mode fibers is used for carrying the fi-
≈ 3 mm diameter beams. Beam splitters split the ref-
nal signal to the electronics since the phase relationship
erence beam into eight beams, one for recombination
between the light of the two chords is established in the
with each probe chord. Each of the reference beams
interferencepatternandcoherenceconcernsarenolonger
pass through another beam splitter, where the recom-
an issue.
bination with the probe beam occurs, and to the final
An IntraAction ATM-1102DA1B Bragg cell beam
Thorlabs fiber coupler, mounted in a simple CP02FP
splitter splits the original laser beam into the reference
fiberport mount. Each probe beam exits the fiber and
beam, now with a frequency shift of 110 MHz, and an
reflects off a turning mirror into the beam splitter con-
unaltered beam, which is then split seven beam splitters
taining the corresponding reference beam. At the final
into the eight probe beams. The Bragg cell power split-
fiber couplers the light containing the interference signal
tingisuneven,makingthereferencebeam≈20mWand
is coupled into multi-mode fibers.
the initial unmodulated beam 140 mW, which is filtered
Since the beams have passed through different fibers,
with a 0.3 OD neutral density filter. After splitting, all
thepolarizationrotatingeffectofthe fibersmustbe con-
beams are attenuated to < 8.3 mW and then routed to
sidered. Light emitted from the fibers is plane polar-
the first set of fiber-couplers, as shown in Fig 1. The
ized, so to maximize the interference present when the
reference beam is coupled, using a Thorlabs PAF-X-18-
probe and reference beams overlap, the polarizations of
PC-A fiber coupler in a K6X kinematic mount, into a
thelightinbothbeamsmustbeparalleltoeachother.7,9
42 m, single-mode fiber and routed directly to the re-
Eachprobechordcollimatorisheldinarotatablemount
combinationoptics. Eachprobe chordis coupledusing a
which allows each probe beam polarization to be inde-
Newport F-91-C1 fiber coupler into a 20 m, single-mode
pendently matched to the reference beam polarization.
fiber. These fibers run from the optical table containing
all previous optics to a 61 cm x 30.5 cm optical bread-
board mounted on a 75 cm flange on the vacuum cham-
B. IF Electronics
ber. The breadboardis mounted with the 61 cm edge of
the board adjacent to the 57 cm edge of a 57 cm x 19.8
cm x 2.54 cm borosilicate window. The Bragg cell (acousto-optic modulator, AOM) op-
The probe fibers terminate at a set of eight Thorlabs eration requires an IntraAction ME-1002 RF generator
CFC-11X-A adjustable focal length collimators in Thor- thatproducesa110MHz,2Wsignal. Thissignalpasses
labs KM100T kinematic mounts. Each collimator pro- through an 8.9 dB directional coupler, transmitting a
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FIG.1. Interferometeroptics layout,showing opticalcomponent layout for allthreefiber-coupledstages: theestablishment of
theprobeandreferencechords,thechord-positioningopticsonthechamber,andthefinalprobe/referencebeamrecombination.
small portion of the 110 MHz signal to the processing nism of attenuation. The laser chords were assumed to
electronics while the rest of the signal is transmitted pass through a circularly symmetric plasma of radius 35
to the Bragg cell. The processing portion of the 110 cm, with an exponentially-decaying radial profile. This
MHz signal is transmitted to a Mini-Circuits ZCS-8-13- plasma is a 2D approximation of the plasma liner ex-
S+ eight-way electronic splitter, creating one 110 MHz pected to be generated by all 30 railguns. The plasma
Local Oscillator (LO) signal for each chord. Each LO maximum and minimum densities were assumed to be
signal is then transmitted to a Pulsar Microwave IDO- 1019 cm−3 and 1016 cm−3 respectively. The simulation
05-412 IQ Demodulator, as shown in Fig. 2. found that laser phase shift is the limiting factor for
The multi-mode fibers carrying the interference sig- the interferometer wavelength selection. Line-integrated
nals terminate at a set of Thorlabs PDA10A photore- electron densities of 1017−1018 cm−2 (corresponding to
ceivers. The output signal from the photoreceivers pass path lengths of 10-20 cm through the plasma, for the
through Lark Engineering MC110-55-6AA bandpass fil- plasma region with densities between 1016−1017 cm−3)
tersof110±55MHztofilterouthigh-frequencycompo- generatedaphase-shiftinexcessofN =15fringes,where
nents of the heterodyne-mixing, electromagnetic inter- N = 15 is an approximate upper limit on the time-
ference from the pulsed-power system, and most low- resolvable number of fringes. At the phase shift limit, a
frequency electrical noise. The filtered signal is trans- 561nmbeamgeneratedalaserdeflectionof<0.5×10−3
mitted to the RF channel of an IQ Demodulator which mandalaserintensityof>95%oftheoriginalintensity.
decomposestheinterferencesignalintotwosignals,Iand Thus, visible interferometry is suitable for density mea-
Q, proportional to the sine and cosine of the frequency surements of the outer regions of the full plasma liner.
difference between the signal frequency and the LO fre-
quency. The I and Q signals pass through a low-pass
filterofmaximumfrequency40MHz,whichremovesany
remaininghigh-frequencynoise. Line-integratedelectron
EarlytestsoftheMarkIplasmarailguns,10 performed
densities ofthe plasmaareextractedfromthe remaining
at HyperV Technologies, indicated a plasma density of
frequencies contained in the I and Q signals. order1017 cm−3 anddiameter5cm,whichcorrespondto
aline-integratedelectrondensityof≈1018 cm−2. These
valuesyieldthesameorderoflineintegratedplasmaden-
III. ANALYSIS OF BEAM PROPAGATION AND LIGHT sities as the outer regions of the simulated plasma pro-
BACKGROUND
file. Thus, visible interferometry should be viable for
single- and several-jet density experiments as well as for
Visible interferometry was chosen by 2D ray-tracing the thirty-jet experiments. This is further confirmed by
simulation which included light deflection, fringe shift, HyperVTechnologies’useofaHeNeinterferometer11 for
and free-free Bremstrahlung absorption as the mecha- their single plasma jet experiments.
4
FIG.2. (a) Anelectrical schematic ofRFcomponentsfor all eight interferometer channels. (b) Alayout of theRFelectronics
for all eight interferometer channels.
IV. EXPERIMENTAL RESULTS effects of bit noise, all signals are smoothed during pro-
cessing using the MATLAB ‘filter’ function to return a
dataarray,Signalnew,whereforeverypointinthearray,
raiPlgLuXnsaitng<le-3je0t0sktuAdioefspweiathktchuerrHeynpteerxVamMianrekjIetplparsompa- Signalnew(n) = N1Σni=n−NSignalold(i), where N = 5 is
the number of points the data was averaged over. The
agation at the lower end of the plasma density range
baseline noise due to the lower-frequency vibrations is
detectable by this interferometer, R nedl ≈ 1015 − subtractedbyusingtheMATLABfunction‘polyfit’tofit
1017 cm−2. Fig. 3 presents interferometry results from
a polynomial function to the signal, excluding the time
shot 30, typical of a “higher” density shot, R nedl ≈ intervalforwhichplasmaispresent,andsubtractingthat
1017 cm−2, and shot 31, typical of a “lower” density
polynomial approximationfrom the entire signal array.
shot, R nedl ≈ 1016 cm−2. Line-integrated electron den-
sities are calculated as R nedl = 4πǫ0mec2/(e2λ0)∆φ. Figs. 3(c) and 3(d) show the line-integrated elec-
As demonstrated in Fig. 3(a) and (b), the interferom-
tron density and phase shift measurements over multi-
eter is capable of the sub-fringe measurements necessary
ple chords, where Chord 8 is 35 cm from the railgun
for these densities. Fig 3(a) demonstrates the raw Q
nozzle and consecutive chords are placed at 6.35 cm in-
signals for Chord 8, for Shots 30 and 31 respectively.
tervals further along the axis of jet propagation. Due to
For Chord 8, the fringe amplitude of the electronic sig-
the curvature of the spherical vacuum chamber and the
nal is approximately +/−120 mV, and the plasma sig-
choiceofdiagnosticports,thischordarrangementhasan
nals, while only a fraction of the total fringe ampli-
averagepath length mismatch of 2 cm between consecu-
tude, are distinguishable from the bit noise as well as tive chords, a cumulative path length mismatch of ≈ 14
the lower-frequency vibrational noise. Figs. 3(b)-3(d)
cm between Chord 8 and Chord 1, and an average path
collectively show that a line-integrated electron density
length mismatch of 20 cm between probe and reference
of R nedl ≈ 1×1017 cm−2 corresponds to a phase-shift path lengths. Limited space on the breadboards at the
of ∆φ ≈ 90o, R nedl ≈ 1 × 1016 cm−2 corresponds to chamberforcesoptics placementwhich canaddupto an
∆φ ≈ 9o, and R nedl ≈ 5×1015 cm−2 corresponds to additional12cm ofpath length discrepancy. Inaddition
∆φ≈4o.
todensityinformation,comparisonofthedensityprofiles
The average electronic signal per chord is approxi- between chords can also yield jet velocity and expansion
mately+/−100mVwithabitnoiseamplitudeof+/−2 data. Shot 30 had a velocity of 15 km/s and Shot 31
mV. Since the raw signals show little noise on the time hadavelocityof26km/s,wherethe velocitiesarecalcu-
scale of the plasma interaction with the interferometer, lated using the time delay between the leading edges of
the limiting signal noise is the bit noise from the dig- the jets, as shown in Figs. 3(c) and 3(d). The multiple
itizers. Thus, the interferometer can detect sub-fringe peaks in each trace are likely due to electrical current
o
shifts of approximately 4 with a signal to noise ratio of ringing in the plasma gun source. Each ring produces a
2 : 1 without signal processing. To further reduce the local density peak in the plasma jet. These features will
5
FIG. 3. (a) A comparison of the raw signal amplitudes of Shots 30 and 31 for signal 8Q. (b) A comparison of the phase
shift amplitudes (in degrees) after background noise has been subtracted between the same chord on Shots 30 and 31. (c)
The line-integrated electron density (left axis) and phase shift (right axis) vs time for Shot 30 and (d) Shot 31 for all eight
interferometer chords, where thechords are designated by their distances from the railgun muzzle.
be investigatedfurtherinother workanddo notdirectly ACKNOWLEDGMENTS
affect the results of this paper.
The authors would like to acknowledge the contribu-
tions from their colleagues Andrew Case, Sarah Messer,
and Samuel Brockingtonat HyperV Technologies,Jason
V. SUMMARY Cassibry at the University of Alambama in Huntsville,
and Thomas Awe, John Dunn, Colin Adams and Josh
Insummary,aneight-chord,heterodyne,visible,fiber- Davis at Los Alamos National Laboratory. This work
optic interferometer achieves sub-fringe resolution even wassupportedbytheOfficeofFusionEnergySciencesof
withpathlengthdiscrepanciesontheorderoftensofcen- the U.S. Dept. of Energy.
timeters between each probe-chord path length and the
1S. C. Hsu, T. J. Awe, S. Brockington, A. Case, J. T. Cassibry,
reference path length. The ability to have path length G.Kagan,S.J.Messer,M.Stanic,X.Tang,D.R.Welch,andF.
discrepancies in the system without signal degradation D.Witherspoon,submittedtoIEEETrans.PlasmaSci.(2011).
allows reduction in the complexity and cost of the inter- 2S.C.Hsu,T.Awe,D.Hanna, C.Adams,J.Dunn, F.D.With-
ferometer by permitting the use of only one reference erspoon, J. Cassibry, M. Gilmore, and D. Hwang, Bull. Amer.
Phys.Soc55,357(2010).
beam for all probe beams. The ability to have path
3F. D. Witherspoon, A. Case, S. J. Messer, R. Bomgardner II,
length discrepancies also makes path length variations M.W.Philips,S.Brockington, andR.Elton,ReviewofScientific
created by the spherical geometry of the vacuum cham- Instruments 80,083506(2009).
ber and limited space for optics a non-issue. The fiber- 4F. D. Witherspoon, R. Bomgardner II, A. Case, S. Messer,
S. Brockington, L. Wu, R. Elton, S. C. Hsu, and M. Gilmore,
optic decoupling of the optics at the chamber from the
Bull.Amer.Phys.Soc55,358(2010).
rest of the interferometer greatly increases the flexibility 5D.KumarandP.M.Bellan,ReviewofScientificInstruments77,
ofpossiblechordbeamarrangements,andthus increases 083503(2006).
the range of accessible experiments. Finally, the sub- 6OxxiusSLIM Plug and Play Datasheet,OxxiusSA(2010).
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line-integrated electron densities down to 1015 cm−2 in Press,NewYork,1970)pp.257–259, 316–323.
additiontothe1017−1018 cm−2 densitiesexpectedfrom 98EP..SHheacjhetn,kOo,pAticpsp,li4etdhOedp.ti(cAsd2d2i,so1n3W(19es8l3e)y., San Fransico, 2002)
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6
10S. Messer, HyperV Technologies Corp., private communication
(2011).
11A. Case, S. Messer, R. Bomgardner, and F. D. Witherspoon,
PhysicsofPlasmas17,053503(2010).