FTS systems with diode based referencing and auto-alignment capabilities
James R. Engel and Rick Dorval
OPTRA, Inc.
461 Boston Street, Topsfield, MA 01983
ABSTRACT
Over the past two decades major advances in FTS have allowed process control engineers to
more readily consider the use of this measurement technique. The most striking advance has been
in the area of data processing facilitated by extraordinary increases in computing power. The
development of improved optical fibers has provided a means for bringing the measurement to
the factory floor while providing a remote "laboratory environment" site for the less-than-robust
spectrometer optical systems. Recent advances in auto-aligned systems again permit
consideration of moving the spectrometer system to locations in close proximity to the process
itself. Generally, these systems are based on the use of HeNe lasers for the reference and autoalign mechanism. This results in large and expensive measurement heads to again argue against
placement of the spectrometers proximate to the process.
This paper describes the successful use of a solid state light source in place of the HeNe laser in
an auto-aligned and referenced FTS system which allows consideration of small and inexpensive
process control spectrometers. A review of a spectrometer system utilizing a combination of autoalign and referencing technologies utilizing diode sources is presented. DOD and NASA support
enabled this dual-use technology to be developed.
Keywords: Fourier Transform Spectrometer, Auto-Align Reference Systems, UV Spectrometer
SYSTEM CONFIGURATION
The spectrometer system is a self-contained assembly shown in Figure 1. The FTS system
specifications are summarized in Table 1. The system is comprised of several subassemblies
which include the thermally and electrically controlled diode laser based probe beam assembly,
auto-align assembly, transducer assembly, exit optics assembly, silicon and photomultiplier
detectors, and the associated electronics. The system was designed to be as modular as possible
such that all the sub-assemblies can be removed and replaced with relative ease. All user
selectable variables are externally accessed via the control panel located on the rear of the
instrument.
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FTS systems with diode based referencing and auto-alignment capabilities
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Probe Beam Optical Assembly
The laser diode assembly shown in Figure 1 consists of a GRIN lens, an optical isolator and a
beam expander. The expanded laser beam is directed through the interferometer with a right angle
mirror toward the beamsplitter/compensator below the clear aperture of the spectrometer. The
probe beam is then split into two linearly polarized components at the beamsplitter. A λ/4
waveplate located in one arm of the interferometer in combination with the mirror rotates one of
these polarizations by 90º. The now orthogonally polarized beams then exit the interferometer
and are directed to the detector assembly with a fold mirror. Contained within this detector
assembly is specially designed Wollaston prism and a PolarcorTM polarizer. This prism
symmetrically introduces a small angle between the two orthogonal polarizations. This angle is
chosen such that, when the polarizations are mixed with the PolarcorTM rotated 45º with respect to
the orthogonal polarizations, the two beams form an interference fringe pattern. This pattern
consists of parallel vertical fringes at a precisely known spacing at the detector plane which are
used in the spatial heterodyning.
Laser diode stability requirements
Substitution of a laser diode for the more commonly used HeNe laser requires addressing several
issues including: coherence length, wavelength stability, and a phase measurement technique.
Diode lasers have more than adequate coherence lengths for use in a W-FTS System, typically
greater than 1meter. Wavelength stability and phase measurement issues, however, require some
new techniques.
Since the location of the sinc2 function (the instrument line shape for boxcar apodization) in
frequency (cm-l) space will be determined by the relationship of the sampling frequency and the
signal frequency shift in the sampling frequency will cause the sinc2 function to change location
in wavenumber space. This is shown in Figure 2 for two arbitrary sampling frequencies: (Note
that for convenience σ1 can be set to zero). If one desires that a change in sampling frequency
will produce a change in reported spectral amplitude of less than 1% a simple spreadsheet model
can be used to numerically derive Table 2 which enumerates the maximum allowed center shift in
the sinc2 (z/2) location to satisfy this condition.
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FTS systems with diode based referencing and auto-alignment capabilities
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The relative spectral shift between the sampling frequency and the spectrum are equal
∆σ(sampling) = ∆σ (spectrum)
σ(sampling)
σ(spectrum)
(1)
For the case where
L = 10 cm
∆σ (spectrum) = 5.5 x 10-3 cm-l,
∆σ (spectrum) = 40,000 cm-l, and
σ(spectrum) = 12,050 cm-l,
∆σ =
σ
5.5 x 10-3 cm-l
40,000 cm-l
=
___∆σ___
12,050 cm-l
(2)
The sampling frequency stability is proportional to the signal frequency shift by the ratio of the
wavelengths (0.25µm/0.83µm)
∆σ (sampling) = 1.66 x 10-3 cm-l
which is the required wavelength stability of the laser.
Thermal and drive current effects
The spectral properties of commercially available laser diodes have been well documented in the
literature123. Because of their broad gain curves (>>10 cm-l) laser diodes require wavelength
stabilization for interferometric applications. The wavelength stability as a function of
temperature and current of a Sharp model LT015MD laser diode was investigated at OPTRA.
1
C. Shin and M. Ohtsu, "Stable Semiconductor Laser with a 7-Hz Linewidth by an Optical-Electrical
Double-Feedback Technique", Optics Letters, Vol. 15, pp. 1455-1457, 1990.
2
E. David and Jean-Made Gape, "Frequency Locking of Laser Diode Using Metallic Vapor Optogalvanic
Spectrum: UI",Appl. Opt., Vol 29, pp. 4493-4498, 1990.
3
H. Furuta and M. Ohtsu, "Evaluations of Frequency Shift and Stability in Rubidium Vapor Stabilized
Semiconductor Lasers", Appl. Opt. Vol. 28, pp. 3737-3743, 1989.
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FTS systems with diode based referencing and auto-alignment capabilities
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The wavelength stability of 0.095 cm-l/mA and 0.94 cm-l/ºC was experimentally determined and
is consistent with that reported in the literature4. In order to achieve 1.66 x 10-3 cm-l stability, the
RSS of the two errors for the current and temperature require that they be controlled to 0.012 mA
and 1.2 m C, respectively. While this is not trivial, it is achievable for short term stability in a
laboratory environment. The drive electronics used to control the laser diode utilize two servo
loops, one to control the temperature (using a thermoelectric cooler) and one to control the drive
current as shown in Figures 3 and 4.
In order to implement the spatial heterodyning technique to determine phase from a single
frequency laser diode, a series of straight line fringes have to be generated. This is accomplished
through the use of a specially designed Wollaston prism. A Wollaston prism consists of two
equal angle calcite prisms optically coupled with optic axes directions orthogonally crossed. The
two output beams are orthogonally plane polarized and slightly unequally deviated
(asymmetrical) with an angular separation. It is this angular separation in combination with a
4
A. Abou-Zeid, "Diode lasers for interferometry," Precision Engineering, Vol. 11 pp. 139-144, July 1999.
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FTS systems with diode based referencing and auto-alignment capabilities
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linear polarizer located 45º to the orthogonally plane polarized beams which causes interference
to occur. The fringe spacing, D, is related to the laser wavelength, λ, and the angular separation
by:
D = λ/(2sinθ)
(3)
where θ is the half the angular separation of the diverging beams. For a laser wavelength of 830
nm this requires and a desired fringe spacing of 75 microns requires an angular separation of
0.63º. Noting that calcite has an ordinary index, no = 1.647237 and an extraordinary index, ne =
1.481564 yields a calcite wedge angle of 1.9º ±10'. The surfaces were polished flat to λ/4 @ 6328
nm, the wedges were bonded with an optical adhesive and the outer surfaces were AR "V"
coated.
The 3-Tri-Element Detector assembly consists of three multi-element arrays each located on the
apex of an equilateral triangle, Figure 5. The detector assembly is positioned such that three
adjacent detectors sample a single fringe at locations separated by 2π/3, Figure 45. Each of the
detector arrays consists of 90 elements which has been designed such that every third element is
wired in parallel. Therefore each detector provides a spatially averaged signal which generates the
I(1), I(2), and I(3) needed for the phase recovery algorithm based on the technique described by
Mertze6. The three signals from each of the detector arrays are then pre-amplified. Error signals
proportional to tilt are generated at every fringe crossing. These three signals are then amplified,
and delivered to appropriate piezo actuators in the auto-align mirror assembly.
5
6
Patent applied for.
L. Mertz, "Complex Interferometry", Applied Optics, Vol. 22, pp. 1530-1534, 1983.
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FTS systems with diode based referencing and auto-alignment capabilities
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Auto-alignment technique
The basic auto-align technique, Figure 7, consists of an expanded laser beam which traverses the
interferometer and falls on a 3-tri-element detector. Each of the tri-element detectors provide the
means for measuring the path length for three paths through the interferometer spatially separated
by 120 degrees. A perfectly aligned interferometer will show three identical path lengths, while
mirror tilt produces unequal path lengths. Each of the three segments path length information and
error signals are generated and applied to piezo-ceramic actuators that sit under one of the mirrors
and allow for tilt correction.
Figure 8 shows additional system elements designed to accommodate phase offsets and amplitude
changes in the laser diode output. Phase offsets between X1, Y1, X2, Y2, and X3, Y3 when tilt error
is zero are very likely to occur due to the mechanical positioning of each of the Tri-Element
detector arrays, to chromatic errors (since the auto-align wavelength is .830µ vs. .25µ wavelength
of interest), as well as electrical phase offsets. The phase offsets of the Tri-Element detector
arrays can be adjusted to ±60° maximum by designating which array outputs are, I1, I2, and I3.
Then instead of measuring Y1, and Y2 directly, the difference between Y1, Y2, and an adjustable
reference voltage will result in error signals proportional to the tilt errors.
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FTS systems with diode based referencing and auto-alignment capabilities
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The concept to handle phase offsets assumes the amplitudes Y1, and Y2 are constant. This may
not be the case. To allow for this, two blocks are added. These circuits use the same sampling
technique described earlier to sample X1, and X2, when Y1, and Y2 = 0 resulting in stored signals
equal to the maximum amplitude of Y (assuming the amplitude of X1 = Y1 and X2= Y2).
SPECTRA
Figure 10 shoes a 0.4 resolution scan of a HeNe laser line source.
ACKNOWLEDGEMENTS
NASA/JPL
OPTRA, Inc
Contract Number NAS7-1109 (SBIR)
FTS systems with diode based referencing and auto-alignment capabilities
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