Proceedings of IBIC2012, Tsukuba, Japan

MOPA42MEASUREMENTS OF MARTIN-PUPLETT INTERFEROMETER LIMITATIONS USING BLACKBODY SOURCE

Abstract
Frequency domain measurements with Martin-Puplettinterferometer is one of a few techniques capable ofbunch length measurements at the level of ~ 100 fs. As
the bunch length becomes shorter, it is important to know
and be able to measure the limitations of the instrument interms of shortest measurable bunch length. In this paper
we describe an experiment using a blackbody source withthe modified Martin-Puplett interferometer that is routinely used for bunch length measurements at the JLab FEL,
as a way to estimate the shortest, measurable bunch
length. The limitation comes from high frequency cut-offof the wire-grid polarizer currently used and is estimatedto be 50 fs RMS. The measurements are made with thesame Golay cell detector that is used for beam measurements. We demonstrate that, even though the blackbody
source is many orders of magnitude less bright than thecoherent transition or synchrotron radiation, it can be used
for the measurements and gives a very good signal tonoise ratio in combination with lock-in detection. We also
compare the measurements made in air and in vacuum tocharacterize the very strong effect of the atmospheric absorption.

MOTIVATIONAt the JLab IR/UV Upgrade FEL facility the bunch
length measurements are made with a modified MartinPuplett interferometer (MPI) [1]. The measurements are
made at full bunch compression at the beam energy of up
to 135 MeV. When the IR FEL is operated with the bunchcharge of 135 pC the bunch is routinely compressed down
to 130 fs RMS. This value assumes that our data evaluation procedure is reasonable accurate. A procedure that
operates in the frequency domain is described elsewhere[2]. A good quantitative agreement between the IR FEL
performance and its 1D model was found when suchbunch length measurements were used as the input to themodel [3]. The other measured beam parameters used as
an input for the model are energy spread, emittance andTwiss parameters at the wiggler. Since the beam parameters other that bunch length are relatively straight-forward
to measure, the agreement gave us some confidence thatthe bunch length evaluation procedure is working reasonably well. However, when UV FEL was commissioned amuch higher small signal gain was measured than the 1Dmodel predicted [4]. When trying to explain the disagreement by hypothesizing, that some electron beam parameters were measured incorrectly, one comes to theconclusion that the error in the measurements would haveto be very large, and only a combination of different parameters measured incorrectly could explain the disa____________________________________________# Pavel.Evtushenko@jlab.org

greement between the measurements and the model. Since
the bunch length is the least straightforwardly measured
beam parameter it drew the most suspicion. The UV FEL
is operated with the bunch charge of 60 pC and bunch
length of 100 fs was measured at full compression.
In addition, the Coherent Transition Radiation (CTR)
source used for the bunch length measurements in the UV
FEL appears to be much brighter that the one used in the
IR FEL. The same Golay cell detector was used with bothbeamlines. Besides the interferometric measurements weminimize the bunch length by maximizing the amplitudeof the Golay cell. The measurements are averaged over
the tune-up beam macro pulse due to the detector’s time
constant. With comparable bunch length, according to the
MPI measurements, we had to reduce the integrated macro pulse charge by a factor of ~ 30, to have the same Golay cell signal amplitude. The macro pulse was shortened
from 250 to 20 µs and the bunch charge was reduced from
135 to 60 pC. Since the power of the coherent radiation is
proportional to the charge squared, the reduction of thecharge actually reduces the CTR power by the factor of~5. One possibility to explain the difference in efficiencyof two setups is the better alignment of the one at the UV
FEL beam line, which we indeed have. Yet, factor of 60
in brightness seemed to be too large to explain only by thealignment. These observation has contributed to the needto evaluate our MPI in terms of its applicability the bunch
length of 100 fs and shorter.
A shorter bunch length corresponds to a broader spectrum and one might expect that the measurements become
easier as the spectrum is affected less by the low frequency cutoff. The bandwidth of the MPI measurements with
CTR is limited on the low frequency side by the size of
the CTR radiator [5], and on the high frequency side by
performance of the freestanding wire-grid polarizer beam
splitter and analyzer. As the frequency becomes higher
the wavelength of the radiation becomes smaller than the
gap between the wires such that the wire-grids performance degrades. Hence, the MPI, whose operation reliesof on the polarization, does not function properly. Therefore, our goal of the interferometer evaluation was tomeasure the high frequency limit of the overall setup.

FREE-STANDING WIRE-GRIDS
The wire-grids, used in our modified MPI, are made ofTungsten wires with diameter of 20 µm with period of 50µm. There are a large number of publications dedicated tothe subject, both theoretical [6,7] and experimental [8-10],
to name just a few. Two dimensionless parameters areusually introduced. The first one is the ratio of the wire
diameter to the grid period 𝑆 = 𝑑 𝑝. The second one is
the ratio of the grid period to the radiation wavelength𝜘 = 𝑝 𝜆. There are several quite different mathematicalISBN 978-3-95450-119-9
Longitudinal Diagnostics and Synchronization1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○P. Evtushenko#, M. J. Klopf, JLab, Newport News, USA

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○MOPA42

Proceedings of IBIC2012, Tsukuba, Japan

approaches to describe electro-magnetic properties of thewire-grids. Some of the approaches (see references in [8])result in relatively compact and simple formulas describing transmission and reflectivity of the wire-grids. These
approaches, however, have limited applicability in termsof S and 𝜘 range. Other approaches that are much moreelaborate mathematically supposedly are applicable for agrid with any S and 𝜘, however, require very involved
calculations. Even more importantly, to our knowledge,
all but one [7] of the models calculates performances of
the ideal wire-grids. That is, the distance between wires isalways exactly the same. It was also demonstrated experimentally that irregularities of the wire spacing affect thewire-grid characteristics rather significantly [10]. It iseasy to see that the wire spacing of the grids used in our
MPI is not uniform. This is one reason why, for now, we
did not want to depend in our conclusions about practicalcharacteristics of the wire-grids on calculations. Also, thegoal of the measurements presented here was not a complete characterization of the wire-grid polarizers but, primarily, measurement of the high frequency cutoff of thewhole experimental setup. It was suggested that, after firstorder measurements are made and the performance of thewire-grids is compared to the CTR spectra, one can judge
if more accurate measurements and more elaborate data
evaluation are needed.

THERMAL RADIATION SOURCE
The source of radiation used in the measurements presented here was a commercially available thermal source
developed for a Fourier Transform Infrared Spectroscopy(FTIR) system [11]. There are several reasons why such asource was used. The first is that such a source is mostaffordable and is easily accessible. Our measurements
were meant to be such. Technical reasons are that for an
ideal blackbody source with sufficiently high temperature
the spectral power density grows monotonically up to afrequency much higher that the frequency range of interest for the bunch length measurements. According to thewell known Plank’s spectral energy densityℎ!

𝑑𝑈 𝑑𝜈 = 8𝜋  𝜈 ! 𝑐 !! ℎ𝜈   𝑒 !!! − 1!!

,a source with temperature of 103 K will reach its maximum power density at about 60 THz. This fits the idea of
our experiment well. Thus we expect a spectrum with thespectral power density growing up to some point. Then, in
the frequency range where the wire-grids polarizers do
not function properly, no interference will occur. At yethigher frequencies the spectrum cannot be measured. As aresult the measured spectral power density will declinewith frequency. Our objective is to compare the highest
frequency observed in the bunch length measurements tothe frequency in the measured spectrum of the thermal
source where the spectral power density starts to decline.
Another important aspect of the blackbody radiation is its
partial temporal coherence of the first order. The coherence time and effective bandwidth of a blackbody with
Plank’s spectral energy density were rigorously calculated[12]. The calculations were made for two different definitions of these quantities introduced by Wolf and Mandel.
It was shown that the two definitions result in comparable
values of the quantities. It is shown for Wolf’s definitions
that the coherence time and the effective bandwidth of theblackbody can be expressed as Δ𝜏! = 0.07  𝛼 andΔ𝜈!"" ≅ 1.37 𝛼, where 𝛼 = ℎ 𝑘! 𝑇, h and 𝑘! are thePlank’s and Boltzmann’s constants and T is the absolutetemperature. Thus one calculates for a blackbody withtemperature of 103 K the effective bandwidth of 28.5 THz
and coherence length 𝑙! = c  Δ𝜏! of 1.01 µm. The important aspect for our measurements is to have a sourcewith a bandwidth wider than the bandwidth of the wiregrid, or else one is just measuring the bandwidth of thesource. The temperature of the thermal source can be optimized with the following rational. On the one hand it
needs to be high enough to deliver enough power in thefrequency range of interest. On the other hand increasing
the temperature further increases the intensity of the radiation at shorter wavelength much more quickly than in
the THz, i.e., FIR where intensity of the source increases
much more slowly. The reason to consider temperature
optimization due to such behavior of the thermal source isthat the very broadband detectors of FIR used for thebunch length measurements are also sensitive to the midIR and visible radiation. When a significant amount of theradiation generated by the source and delivered to thedetector is not polarized by the wire-grids, it is seen in themeasurements as a large background and the interferogram appears to be just a small perturbation in the measurements. To mitigate this effect the input window of thedetectors was covered with black polyethylene, whichvery effectively cuts off the visible and mid-IR radiation.

EXPERIMENTAL SETUP
At the JLab FEL facility one of the user labs is setup forexperiments with the broadband coherent synchrotron
radiation. The radiation is transported [13] to the lab fromthe exit of the last bunch compressor dipole, where thebunch length reaches its minimum. The optical table in
the lab, where radiation is delivered, is equipped with twointerconnected vacuum chambers and in vacuum FTIR.
The vacuum chamber where different user experiments
can be setup is large enough to accommodate the modified MPI as well as the thermal source. The pressure in
the chamber during measurements was 100 mTorr or less.
The thermal source radiation was collected and collimated by a 90 degree off-axis parabolic gold-coated mirror with a diameter of 2 inches. The mirror was set up to
have its focus on the source filament. The visible radiation emitted by the source was used for the initial alignment of the setup. Between the source and the parabolic
mirror, a mechanical chopper was installed. The reasonsfor using the chopper are a. the interferometer itself is a
step scan device and works with amplitude modulated
radiation only, b. the Golay cell detector is also a pulsed
device that needs amplitude-modulated signals, and c.
due to the low power level of the source and the use ofISBN 978-3-95450-119-92Longitudinal Diagnostics and Synchronization

Proceedings of IBIC2012, Tsukuba, Japan

MEASUREMENTS RESULTS
As mentioned earlier at the JLab FEL the bunch getscompressed down to about 100 fs RMS. Fig. 1 shows oneof the CTR power spectra that according to our data evaluation corresponds to the RMS bunch length of about 110
fs. The analytical fit function used for the data evaluation
with the parameters obtained by non-linear least square fit
is shown as well. Spectrum of the thermal source measured with the same instrument is also shown in Fig. 1. The
shape of the thermal source spectrum qualitatively agrees
with our expectations presented above. The measured
spectral density of the source starts to decrease at about
2.7 THz. At that frequency the CTR spectral density is
reduced to the level of 5 % from it maximum. Our interpretation of the data shown the Fig. 1 is that when the
instrument is used for an bunch length measurements at
the level of 100 fs RMS, the measurements are not limitedby the bandwidth of the interferometer. However, it is still
possible that the beam measurements and their data evaluation are affected by the transfer function of the instrument. We do not consider these measurements as a comprehensive and final study of the transfer function, butrather as a demonstration that it can be made with a simple thermal source and, very importantly, room temperature detector – the same detector as used for the beammeasurements. One way to estimate the bunch length,
which would correspond to the bandwidth of the MPI is to
use the bunch length evaluation procedure, normally applied, to the CTR spectra, to the spectrum of the thermal
source. The result of this is ~ 47 fs RMS bunch length.
This means that, for an electron bunch length near 50 fs,
the bunch length measurements will be strongly affectedby the bandwidth of the instrument unless the data evaluation takes the real transfer function of the MPI in to

Figure 1: CTR and thermal source spectra.
account. Note that wire-grids with wire diameter and period up to four times smaller than ones we used are commercially available. Therefore, it might be possible toextend the applicability of the MPI for the bunch length
measurements down to at least 25 fs RMS or smaller,
dependent on the detectors bandwidth.
The CTR spectrum shown in Fig. 1 was measured in air
at atmosphere pressure, while the thermal source spectrum was measured in vacuum. We have repeated themeasurements with the thermal source in air to demonstrate the magnitude of the atmosphere absorption. Two
spectra of the thermal source measured in vacuum and in
air are shown in Fig. 2, where vast amount of absorption
lines in the frequency range of interest can be seen. The
absorption lines do not appear in the CTR spectrum explicitly because such measurements are usually made withvery poor frequency resolution to save time. This does notmean, of course, that the CTR measurements are not affected by the atmosphere absorption. Ideally, such setups
should be used in vacuum. It is also interesting to see theeffect of the atmosphere absorption in the time domain,i.e., on the interferogram itself. Figures 3a and 3b show
two interferograms measured in vacuum and in air.

Figure 2: Thermal source spectra measured in air andvacuum by Golay cell and D201 pyro-electric detector.
ISBN 978-3-95450-119-9

Longitudinal Diagnostics and Synchronization3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○room temperature detector, a lock-in detection technique
had to be used, thus requiring modulation of the signal. A
modulation frequency of 10 Hz was used. A Stanford Research System lock-in amplifier SR850 was used for themeasurements. The amplifier was typically setup with 80
dB gain, time constant of 300 ms, and 12 dB/octave filter.
The level of the Golay cell signal at the modulation frequency was on the order of 100 µV. The amplitude of thesignal measured with the lock-in amplifier was sent to oneof the outputs of the amplifier and was measured with adigital oscilloscope. A LabVIEW application controls theoscilloscope, downloads its waveforms and measurements
at the rate of a few Hz and broadcasts the results over the
Ethernet. Another LabVIEW application originally madefor the bunch length measurements with the MPI was
modified to use the data of the lock-in amplifier. To improve the signal to noise ratio we usually average over 4
to 16 measurements at each position of the scanning mirror. This in a conjunction with the 300 ms time constantand large number of points measured for each interferogram resulted in a relatively long one hour order of magnitude measurement time. This did not present any problems due to the high degree of overall automation.

MOPA42MOPA42

Proceedings of IBIC2012, Tsukuba, JapanACKNOWLEDGEMENT
Authors would like to thank Dr. Nick Agladze from
Cornell University for numerous insightful discussions
and for landing the D201 pyro-electric detector for themeasurements.

REFERENCES

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 3a: Interferogram – thermal source in vacuum.

Figure 3b: Interferogram – thermal source in air.
While the in-vacuum interferogram has essentially only
central burst, the interferogram measured in air has a verylarge number of oscillations extending to a very large path
length difference.
Golay cell detectors are frequently used for measurements with MPI and are considered to have relatively flat
wavelength response due their operational principles. Pyro-electric detectors are an alternative to the Golay cellwith the price considerably lower but with possibility ofstrong dependence of the sensitivity on the wavelength.
With the experimental setup described above it is straightforward to compare performance of the Golay cell andother detectors. Making measurements in vacuum wehave compared the Golay cell and the FIR pyro-electricdetector D201 made by Bruker. Results of the measurements are shown in Fig. 2. The two detectors perform
quite similarly. While the D201 might be somewhat lesssensitive in the frequency range up to about 3 THz it is
not showing any very strong oscillations in its response.
From this we can conclude that such detectors can be an
attractive replacement for the Golay cell detectors whenapplied to the bunch length measurements.

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[2] P. Evtushenko, “Electron Beam Diagnostics at the
ELBE Free Electron Laser”, Ph.D. Thesis TU Dresden, Dresden, Germany, 2004
[3] S. Benson, Private communication[4] S. Benson, et al., “Beam line commissioning of
UV/VUV FEL at Jefferson Lab”, Proceedings of
FEL2011, Shanghai, China, 2011[5] S. Casalbuoni et al., “Far-Infrared Transition anddiffraction Radiation”, TESLA Report 2005-15[6] W. G. Chambers et al., “Theory of the scattering ofelectromagnetic waves by a regular grid of parallel
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[9] W. G. Chambers et al., “Characteristic curves for the
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[10] C. L. Mok et al., “The far-infrared performance andapplication of free-standing grids wound from 5 mmdiameter Tungsten wire”, Infrared Physics, Vol. 19,
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4Longitudinal Diagnostics and Synchronization