CHAPTER THREE SPECTROMETER DEVELOPMENT
START
Enter user details, date, scan details, (for associated text file),
and file name of first scan. This should end with a number
that is then iterated in consecutive scans.
Set-up
Scan
Scan
Scan spectrum,
re-set instruments,
or quit?
Main Menu
Scan spectrum,
set-up instruments,
or quit?
Set-up
Set-up
LIA
SFS
Laser?
Quit
User sets:
 single or multiple scan, (scan
consecutive frequency ranges
then link them together to
give a single scan, or scan a
single frequency range a
number of time to improve
the S:N ratio).
 dwell time per step in scan
 scan range
 x-axis scale spectrum
frequency either recorded as
upper sideband, lower
sideband or microwave
frequency
new Scan or Quit
Yes
User can:
 save spectrum
 print spectrum
 expand areas of interest
 measure peak frequencies
Quit
STOP
SCAN
display data on PC
as recording
Abort
Scan
No
End of Scan
On LIA:
 User sets sensitivity, PSD
phase, ref. frequency
harmonic, time base.
 VI pre-sets dynamic reserve,
bandpass filter, offset,
dynamic range.
On SFS:
 User sets analogue/digital
scan, no. of steps in scan,
FM/AM modulation settings,
Frequency pre-sets F1-F9,
F, microwave power.
 VI pre-sets RF settings during
scan, power levelling.
On Laser:
 User enters information on
laser settings e.g. power
output, laser gas. User MUST
enter FIR laser frequency or
wavelength before continuing
as this is used to calibrate the
frequency scale on each scan.
No
Yes
Accessed set-up
menu from scan
menu?
Figure 3.20: Flow diagram showing the breakdown of each function in the TuFIR
Control Programme into smaller tasks.
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CHAPTER THREE SPECTROMETER DEVELOPMENT
QUIT
Scan
Experimental
Set-up
Laser
Set-up
SFS
Set-up
LIA Set-up
Experimental
Set-up
Start
Scanning
Return to
Scan Menu
Abort Scan
Figure 3.21: Menu structure of the TuFIR control programme as seen by the user (for
illustration only). Black arrows indicate the obligatory set-up procedure at the start of
the scan, red arrows show the remaining screen order in each menu option.
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CHAPTER THREE SPECTROMETER DEVELOPMENT
Figure 3.22: The hierarchical structure of the TuFIR Control Programme designed to
control the TuFIR experiment. The top level VI is on the left, and the red lines illustrate
links between lower level subVI’s. Each VI is represented by its icon. The ‘spectrumview.VI’ is circled.
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CHAPTER THREE SPECTROMETER DEVELOPMENT
The hierarchical structure behind the Cambridge TuFIR Control Programme is
shown in figure 3.22. With over 60 subVI’s, many of which are reused in higher level
VI’s, the structure is slightly complicated! Over 40 of the VI’s in this programme were
written in full by the author, 16 were modified from the SFS and LIA ID’s, and the
remainder were intrinsic LabView VI’s, taken directly from the software libraries. It is
not feasible to reproduce all the programming details in this thesis.
Once the user has set-up the instrument and scan parameters, the most significant
part of this programme is the scan routine itself. The block diagram for the spectrum
view.VI is shown in figure 3.23. This VI controls the data acquisition cycle for a single
scan over one particular frequency range. Initially the programme sets up an empty 2D
data array with two columns (frequency and intensity) and the same number of rows as
points in the spectrum. The GPIB initialises the bus and checks that two way
communication has been established between the computer and each of the instruments.
In parallel with these two processes a sub-VI reads all the frequency points in the scan
from the SFS. The scan range is expressed in terms of the microwave frequency at the
SFS but can be adjusted in other VI’s to reflect the true upper or lower sideband
frequency. The SFS will only scan up its frequency range so the scan routine only ever
operates unidirectionally. The microwave power is switched on at the lowest microwave
frequency to record the first point. The programme waits for 15msec plus half the dwell
time per step to ensure that the microwave frequency has stabilised. During this time it
records the first frequency reading into the top row of the data array, (figure 3.23a). The
programme then sequences to the second data acquisition stage. It then waits again for
half the dwell time per step, to ensure that the signal response at the LIA actually
originates from the particular sideband frequency at this point in the scan. If the LIA
response is not matched to the signal frequency the line profile will be distorted and the
transition frequencies will appear to shift in the final spectrum. These effects are
observed if the dwell time per step is set too low or differs greatly from the time-base on
the LIA. Another sub-VI obtains five voltage readings from the LIA, each measured 1/5th
of the dwell time apart. The voltage that is read into the spectrum view.VI is actually an
average of these values. This average is read into the data array intensity column, (figure
3.23b). The premise of the data smoothing is to reduce the noise level without biasing the
true signal value [23]. The simple ‘data smoothing’ was included in this programme
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CHAPTER THREE SPECTROMETER DEVELOPMENT
a. ‘true’ case in first sequence frame.
Refers to the very first frequency point in
the spectrum
b. Second sequence frame.
Collects readings from the LIA
c. Block diagram including ‘false’ case in first sequence
frame. Collects readings from the SFS.
Figure 3.23: The block diagram for the spectrum view.VI. This programme controls the
data acquisition for a single scan over a single frequency range. In ‘G’, the Boolean cases
and sequences are overlaid, so certain sections of the whole block diagram have been
reproduced in parts a. and b. (see text for details).
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CHAPTER THREE SPECTROMETER DEVELOPMENT
to eliminate the effects of random noise and FIR laser frequency drift at each point in the
spectrum. The programme triggers the SFS to switch to the next frequency point in the
scan and the whole acquisition cycle is repeated. The data acquisition cycle is enclosed
in a ‘while loop’ that executes an equivalent number of times to the number of points in
the scan. The final data array from this programme is read into the interactive view.VI,
where the ‘raw’ data is saved as a spreadsheet file ***.dat and printed. The control
programme is not used process the TuFIR spectra further. The ‘raw’ data file is usually
imported into a standard graph package, such as Microcal Origin, for this purpose. The
spectrum view.VI was modified to construct the multiple scan routines. The ‘wide range
coverage spectrum.VI’ executes one complete scan as described above, over a pre-set
frequency range, F1 to F2, then shifts the scan range to F2 to F2+(F2-F1). It can make up to
ten such consecutive scans, and then links the data together to give a single large data file
and one single spectrum, (figure 3.24).
Upper Sideband
Frequency (GHz)
867.80
1.2
936.25
867.75
936.30
867.70
936.35
867.65
936.40
867.60
936.45
936.50
867.55
Lower Sideband
Frequency (GHz)
1.0
0.8
Intensity (arb units)
0.6
0.4
867.5473GHz
936.2686GHz
0.2
0.0
-0.2
-0.4
-0.6
491,49 480,48
98,2 87,1
-0.8
-1.0
34.20
34.25
34.30
34.35
34.40
34.45
34.50
Microwave Frequency (GHz)
Figure 3.24: A ‘wide range’ frequency spectrum, covering two rotational transitions
in SO2. The spectrum was constructed from 3 separate 100MHz scans. As the FPI
plates had been removed both the Upper and lower sideband transitions were
observed (colour coded for clarity). The signal was FM modulated at 75kHz.
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CHAPTER THREE SPECTROMETER DEVELOPMENT
The ‘average s/n view.VI’ repeatedly executes the scan routine over one fixed frequency
range. In each consecutive scan, the frequency and intensity data are summed then the
frequency data is averaged at the end of the final scan. In each individual scan, the signal
intensity remains approximately constant, but the noise is varying randomly. Assuming
that the measured noise intensity at each point in the spectrum is governed by a Poisson
Distribution, the S:N ratio in these spectra will be improved by a factor of n in
comparison to a single scan (where n is the number of times the scan sequence is
repeated) [23]. This is illustrated in figure 3.25.
1.0
3 scans
single scan
0.8
Intensity (arb units)
0.6
897,1
998,2
<2
1
8
0.4
7
0.2
0.0
x 400
-0.2
-0.4
-0.6
-0.8
-1.0
936.24
936.25
936.26
936.27
936.28
936.29
936.30
Upper Sideband Frequency (GHz)
Figure 3.25: The n improvement to the S:N ratio between single and multiple scans.
The lineshape has not been distorted by the summation, and the inset shows that the
noise has been averaged out to a lower level than in the single scan
3.5 Conclusion
The development work and modifications undertaken by the author significantly
improved the performance of the Cambridge TuFIR Spectrometer. All these changes are
summarised in Table 3.5. In configuration B, with the FPI plates out, the spectrometer
sensitivity was improved by at least two orders of magnitude in comparison to the
original spectrometer configuration, (A). Consequently, TuFIR spectra could be recorded
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CHAPTER THREE SPECTROMETER DEVELOPMENT
Instrumentation
Optical Bench
Laser System
Modifications and Improvements











Optical System




Diode System
Microwave System









Absorption Cell
Detector System









Data Acquisition

entirely rebuilt system on a new custom made optical bench
switched pump-FIR laser orientation from 180o to 90o
aligned whole system
established vacuum integrity in both lasers
replaced BW’s and steering optics in CO2 laser cavity
re-coated FIR laser mirrors
removed 1 mirror from pump laser path
reduced pump laser pathlength
incorporated re-circulating chiller onto FIR laser
positioned water filter on CO2 laser cooling to improve
cooling water flow
incorporated needle values onto FIR laser for flowing gas
operation
re-arranged optical system to reduce the number of times the
FIR and TuFIR beams were reflected
novel use of grid polariser to direct TuFIR beam
re-coated all mirrors in gold
mounted all optics in proper stands (not hanging from
clamps!)
switched diplexer exit port from 4 to 1
increased r.o.c. on both focusing mirrors
repositioned FPI after absorption cell
replaced FPI plates with new mesh
coated internal faces of FPI with gold
obtained sidebands from new diode mixer for the first time
replaced battery in bias supply
obtained sidebands with microwaves from the SFS for the
first time
replaced coax link between SFS and diode to reduce power
losses
switched from double to single pass
re-designed cell with Brewster Windows
designed cell mounts to support cell from underside
identified pre-set scan parameters for optimum signal
intensity
raised FM modulation frequency
removed FPI plates to improve sensitivity by 1 order of
magnitude
replaced internal dewar strutts that fix He can in position
replaced batteries in pre-amp
designed translation stage and mount for detector
wrote and commissioned novel control and data acquisition
system
Table 3.5: A summary of the modifications made by the author
to the Cambridge TuFIR Spectrometer
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CHAPTER THREE SPECTROMETER DEVELOPMENT
at much higher S:N ratios, and the spectrometer could be used to detect a wider range of
spectra, particularly those from transient species. The recent introduction of the Brewster
Window cell improved the spectrometer sensitivity again by one order of magnitude.
With the new microwave synthesiser, the nominal precision of the instrument is limited
only by the uncertainty in the FIR laser frequency. Typically this is between 300 and
500kHz [7]. If the FIR laser was cooled and aligned its frequency did not drift over a
single scan. Therefore, the spectral precision could be improved by recording a reference
transition in conjunction with the spectrum of interest. FIR calibration tables, with a
published frequency accuracy of at least 100kHz, exist for this purpose [24]. Since the
spectral resolution was ultimately limited by the intrinsic linewidth of the absorbing
species, it was not improved any further by the preceding modifications. No additional
instrumental broadening was observed on the TuFIR spectra provided that the scan
parameters were set correctly.
As a result of all these modifications, the TuFIR spectrometer was equipped to
search for new transient species, or record highly accurate pressure broadening data. The
results presented in the next two Chapters describe such studies.
106