6. Measurements of Instrument Line Shape and Verification

The underlying hypothesis of this chapter is that simultaneous measurement of a very narrow emission or absorption line together with the solar absorption spectrum will provide a direct measurement of the instrument line shape and therefore remove one major uncertainty from the measurement process. Such a line was obtained from the infrared laser system (section 5.1). These measurements were validated by comparing the absorption spectrum of a hydrogen bromide line from the low pressure cell (appendix A2.1) with a simulated line obtained by convolving the measured laser line with a theoretical, Doppler broadened hydrogen bromide line (appendix A3). A direct measurement of the instrument line shape with the cell is not possible for high resolution instruments as the Doppler width of the hydrogen bromide line is comparable to the resolution of the instrument, but it does provide a useful consistency check on the laser measurement.

6.1 Experimental Setup Used for Line Shape Measurements.

The experimental layout used to record solar spectra simultaneously with the infrared laser and the hydrogen bromide cell is shown schematically in figure 6-1.

The infrared laser system is positioned alongside the scan arm of the spectrometer whence its output beam may be directed onto the beam splitter by a plane folding mirror. The beam splitter combines the laser with the solar beam, directing it into the spectrometer where the combined beam passes through the hydrogen bromide cell before entering the fore-optics and interferometer.

Figure 6-1. Experimental layout for simultaneous solar, laser and HBr measurements. The IR laser system is positioned alongside the scan arm of the interferometer with a plane folding mirror and beam splitter to combine the two beams, and the HBr cell inserted in the interferometer's sample compartment (section 6.1).

The primary function of the National Physical Laboratory's Bruker 120M is as a 'mobile standard' used to ensure consistency between Fourier spectrometers permanently deployed at NDSC sites and mobile instruments used as part of the NDSC. This commitment has meant that the measurements described below were made during two of these intercomparisons rather than at the NPL in Teddington.

6.2 Measurements Made at Table Mountain, California.

The NPL's Bruker 120 M, along with the instrument line shape equipment described above, were deployed at the Jet Propulsion Laboratory's Table Mountain Facility (117º 35' W, 34º 36' N) from 29th October until 16th November 1996 for the purposes of intercomparison with the Bruker 120 M of the National Centre for Atmospheric Research and the Jet Propulsion Laboratory's Mark IV balloon-borne spectrometer (JPL [2000a]). NASA's ATMOS instrument (JPL [2000b]) was also included in the intercomparison although not part of the NDSC.

The situation was not ideal for the work described here as there was effectively no means of controlling the temperature of the laboratory. The instruments were set up in an uninsulated, high-roofed shed with one large fan heater which, when used, caused considerable draughts of varying temperatures; outside temperatures ranged from -15 ºC to +30 ºC. These factors seriously degraded the frequency stability of the laser.

The upper plot in figure 6-2 shows a series of five successive measurements of the laser line taken on 3rd November using the experimental set up described in section 6.1. The series illustrates well the problem of frequency drift in the laser which was encountered throughout the intercomparison. The leftmost solid line shows the first of the five measurements with subsequent scans lying in order to the right of this. The peak height can be seen to fall of due to the laser frequency drifting away from the centre of the gain curve to where it was initially tuned. This instability not only made reliable line shape measurements difficult to obtain at any given time, but also hampered the alignment of the laser system to the spectrometer. As described in section 5.5, this relies crucially on the laser output remaining constant throughout the alignment procedure. It is clear from figure 6-2 that this was not the case as the strength of the line can be seen to change as the oscillating mode drifts across the gain curve. Overlay plots of the P8 lines of each of the isotopes of hydrogen bromide, made simultaneously with each of the above measurements, are shown in the lower plots as evidence that the movement in the laser line is not due to drift in the frequency calibration in the spectrometer.

Figure 6-2. Spectra illustrating frequency drift in the IR laser. The upper plot shows five consecutive laser spectra recorded as the laser frequency drifted away from the centre of the gain curve. The lower left plot shows the P8 line of H⁸¹Br and the lower right the P8 line of H⁷⁹Br recorded simultaneously with the laser (section 6.2).

Some of the spectra recorded at Table Mountain, exhibit brief periods of laser stability. This occurred when the thermal drift neared a turning point and the rate of change of the length of the cavity decreased and stopped momentarily before increasing again in the opposite direction. Figure 6-3 shows overlay plots of three micro-windows from three such measurements taken on 3rd November; no drift is evident in these plots as the centre lines of the three measurements overlie each other. The upper plot shows the laser line while the lower left plot shows the P8 line of H⁸¹Br, and the lower right one the same line of H⁷⁹Br. Although there is no obvious asymmetry in the hydrogen bromide lines, it is clearly visible in the laser, with the first minimum on the high frequency side of the laser line centre lying above the zero intensity line (note that two of the laser spectra are indistinguishable in this region).

Figure 6-3. Micro windows from three consecutive measurements with IR laser stable. The top plot shows three consecutive laser spectra (two of the spectra are indistinguishable in this region) while the lower left plot shows the P8 line of H⁸¹Br and the lower right one the P8 line of H⁷⁹Br recorded simultaneously with the laser. Although there is no obvious asymmetry in the hydrogen bromide lines, it is clearly visible in the laser lines (section 6.2).

Each of the hydrogen bromide lines were fitted both with two simulated lines, one constructed using the laser line from that scan, and one based on the theoretical line shape for the instrument. The simulated lines were constructed by convolving the instrument line shape (either theoretical or simultaneous laser measurement) with the simulation of the HBr absorption feature.

Figure 6-4 shows the results obtained when H⁷⁹Br and H⁸¹Br were fitted with simulated measurements based on the theoretical instrument line shape, and in figure 6-5 using the laser measurement. The depth of the simulated absorption line, its precise frequency and the intensity of the background radiation in that micro-window were the fitted parameters. Fitting was carried out using a routine written in Matlab based on the 'leastsq' non linear least squares minimisation function with 'best fit' being determined using its standard termination criteria (MathWorks, Inc., [1996]). The left hand pair of plots in each figure shows the results for H⁸¹Br and with the results for H⁷⁹Br on the right. The lower, larger axes in each figure contain the actual fits of the lines while the smaller, upper axes display the differences between the simulations and the measurements. It can be seen in figure 6-4 that there are large, approximately symmetrical, features in the residuals in the vicinity of the line in the case of the theoretical line shape, indicating that the measured line shape is somewhat broader than the theoretical value. When the simultaneous laser measurement is used in place of the theoretical line shape (figure 6-5) a broader simulation results but a large asymmetric component becomes obvious in the residuals of the fit near the absorption lines. Possible causes of this asymmetry shall be discussed in chapter 7.

Figure 6-4. Fits of simulated (using theoretical ILS) P8 HBr lines (solid) to measurements (dots); upper axes show difference between synthetic and real data (section 6.2).

Figure 6-5. Fits of simulated (using laser measurement of ILS) P8 HBr lines (solid) to measurements (dots); upper axes show difference between synthetic and real data (section 6.2).

6.3 Measurements Made at Lauder, New Zealand.

The NPL's Bruker 120 M, along with the instrument line shape equipment described in chapter 5, were shipped to the NDSC site at Lauder (169º 41' E, 35º 3' S) from 9th February until 21st February 1997 for the purposes of intercomparison with the National Institute of Water and Atmospheric Research's Bruker 120 M.

The experimental set up used for instrument line shape measurements was identical to that used during the intercomparison at Table Mountain (section 6.2) except for the addition of the thermocouples (section 5.2). The spectrometer sustained significant damage to its electronic filtering systems and beam splitters in transit and concern has also been expressed that the the solar tracker used at the intercomparison (supplied by NIWA) may have been responsible for increased noise on the spectra. Although laser spectra were recorded at Lauder none are presented in this section as they are of poor quality and do not contribute to the understanding of the issues under consideration beyond that which was established from the work at Table Mountain.

The temperature of the laboratory in which the instruments were set up was controlled such that the recorded long-term drift varied between 0.04 K/hour and 0.09 K/hour over a ten hour period during the day. Superposed on this were short-term oscillations of between ñ 0.9 K with a 1 hour period and ± 0.5 K with a 25 minute period . Although the inner and outer boxes of the laser system very significantly reduced the amplitude of these oscillations, they did not totally isolate the laser cavity from the temperature fluctuations in the laboratory. Temperature fluctuations were detected by the thermocouple attached to a spacer bar which were similar to those outside the system box. These had the same period as the fluctuations in the laboratory temperature but had a time lag of some nine minutes (figure 6-6). The upper plot shows a ten-minute moving average of the temperature record from the laser cavity while the lower plot shows the corresponding ten-minute moving average of the laboratory temperature. In both plots the long term rise in temperature has been removed to facilitate comparison of the short term fluctuations. Although the signal to noise ratio of the cavity temperature trace is poor, periodic variations corresponding to those in the laboratory temperature can clearly be seen.

Figure 6-6. Plots of temperature data (10-min., moving averages) from Lauder with long term changes removed. Upper plot shows small scale fluctuations in cavity temperature driven by fluctuations in laboratory temperature (lower plot); a time lag of some 9 minutes can be seen between the two (section 6.3).

6.4 Conclusions.

The standard experimental layout for recording combined solar, laser and hydrogen bromide spectra has been described. Laser and hydrogen bromide spectra recorded at Table Mountain, California, have been presented along the results of attempts to fit the measured hydrogen bromide lines with simulations based on the laser measurements and on calculated instrument line shapes.

Laser system temperature and power data recorded at Lauder, New Zealand, have been presented and show evidence of temperature drift in the laser cavity giving rise to frequency drift and mode hopping in the infrared laser; this shall be further considered in chapter 7.

Summary
Acknowlegements
Contents
Chapters: 1, 2, 3, 4, 5, 6, 7, 8
Appendices
References