chapter three
advertisement
CHAPTER THREE SPECTROMETER DEVELOPMENT CHAPTER THREE Development of the TuFIR Spectrometer Three characteristic properties are used to judge the overall performance of any spectrometer [1]: 1. sensitivity, 2. resolution, 3. precision. All three parameters are closely related, and it is not usually possible to change one without affecting the others. For high resolution, gas-phase, FIR spectroscopy, the sensitivity, resolution, and precision should all be as high as possible but in most experiments a compromise is reached. The Cambridge TuFIR spectrometer has been developed with two major applications in mind: 1. to search for the spectra of novel transient species, 2. to record highly accurate pressure broadening data. In the first case, the initial radical searches are made at very high sensitivity, over a wide frequency range at the expense of the spectral resolution and precision. Once the radical spectrum has been observed, its S:N ratio is optimised, essentially making the spectrometer even more sensitive. The scan windows are then reduced and the radical spectrum is re-recorded with high precision and resolution, but at a lower S:N ratio. Consequently, a definitive assignment of the spectral features is possible. In the pressure broadening experiments, the sensitivity and precision have to be optimised so that the line profile can be recorded as accurately as possible. Many pressure-broadening studies concentrate on single, isolated spectral lines, so the spectral resolution is not as significant. In addition, the resolution changes through the experiment so it can sometimes be advantageous not to resolve all the components of a spectral feature, provided that the true line-profile can be modelled correctly. This is illustrated in Chapters 4 and 5 of this thesis, where pressure-broadening studies are discussed further. 65 CHAPTER THREE SPECTROMETER DEVELOPMENT The sensitivity of the TuFIR spectrometer is directly related to the minimum detectable signal power. Generally, this depends on the responsivity and noise characteristics of the detector (related to the S:N ratio of the system), the fraction of the incoming radiation absorbed by the sample (governed by Beer Lambert’s Law), and the electronic signal processing. For a bolometric detector with uniform responsivity over the filter bandwidth, , Philips has shown that the S:N ratio is given by [2]: S PTuFIR t (3.1) N NEP where PTuFIR is the power of the TuFIR sideband at the detector, the NEP is the noise equivalent power of the detector, and t is the integration time at each point in the spectrum. Although the TuFIR sidebands are more intense than the blackbody background radiation, the source noise does not usually exceed the detector noise, (figure 3.1). The spectrometer sensitivity is therefore directly dependent on the power of the TuFIR sidebands. In this thesis, the sidebands are discussed in terms of their intensity (which is proportional to PTuFIR) and are expressed in terms of their video-detected voltage, measured either at the oscilloscope or the LIA. For any transition at frequency o between two non-degenerate energy levels, i and j, the sideband intensity is given by Beer-Lambert’s law [3]: V Vo expS ij NLs( o ) (3.2) where V is the sideband intensity at the detector (in Volts), Vo is the sideband intensity entering the cell (in Volts), Sij is the transition line strength, derived from the quantummechanical transition probability, N is the molecular concentration of the sample (in molec.cm-3), L is the sample length (in cm), and s(-o) is the transition profile, where is the sideband frequency. For an optically thin sample, equation 3.2 can be rewritten as: V Vo 1 S ij NLs( o ) so the fractional sideband absorption is given by: (3.3) V Vo V S ij NLs( o ) (3.4) Vo Vo As the sidebands are either AM or FM modulated, the fractional sideband absorption is time and frequency dependent and the actual sideband intensity at the LIA depends on the harmonic settings of the PSD. Consequently, the spectrometer sensitivity differs between 1st and 2nd derivative spectra, and can be affected by many of the scan parameters. In this thesis, the sensitivity is either quoted in terms of the percentage 66 CHAPTER THREE SPECTROMETER DEVELOPMENT noise spectrum – 0.008 lasers off noise spectrum – lasers on Intensity (arb units) 0.006 A noise spectrum – 0.004 CO2 laser de-tuned 0.002 B noise spectrum – B B B B FIR laser de-tuned B 0.000 B -0.002 B 700.4 700.6 700.8 701.0 701.2 B 701.4 Frequency (GHz) Figure 3.1: Noise Spectra, recorded on the Cambridge TuFIR spectrometer. The top two plots verify that the source noise does not exceed the detector noise provided that the laser system is tuned correctly. If the CO2 laser is de-tuned, large spurious noise peaks appear (A). If the FIR laser is de-tuned, sharp interference peaks appear in the spectrum (B). Blake et al have shown that these peaks are due to interference between the FIR laser frequency, and ‘feedback signals’ from the diode mixer e.g. TuFIR and FIR frequencies [1]. These peaks have random phase and their frequency spacing approximately matches the free spectral range of the laser. 67 CHAPTER THREE SPECTROMETER DEVELOPMENT absorption of the sideband (V/Vo x 100) or N/V, the number of molecules per cm3 that can just be detected above the noise level. The S:N ratio is often used to compare the relative sensitivity between two similar scans, covering identical transitions from the same molecule. Rayleigh’s Criterion expresses the spectral resolution of a system. Two spectral features of equal intensity, Imax, centred at frequencies and +d, are just resolvable from each other when the intensity between the peaks is [4]: I 2 d 2 8 2 I max (3.5) If d/ is very small, the instrument operates at high resolution. In visible and near-IR spectroscopy, the spectral resolution is often limited by the bandwidth of the source [5]. The linewidth of the TuFIR source is determined by the stability of the FIR laser, as the microwave source is accurate to less than 10Hz [6]. The bandwidth of most FIR laser lines is between 300kHz and 500kHz, and depends on the alignment and fine-tuning of the whole laser system [7]. However, in FIR spectroscopy the spectral resolution is determined by the intrinsic line profile of the absorbing species, i.e. s(-o). The total line profile is described by the Voigt function, a convolution of the Doppler-broadening and pressure-broadening contributions to the lineshape. Over the range of this spectrometer, (600GHz to 1.2THz), the Doppler-broadening of a ‘medium weight’ molecule, such as SO2 or CHF3, ranges from around 500kHz to 1.2MHz at 298K. The pressure broadening can be reduced by operating at low pressures, but generally exceeds the Doppler linewidth, even at sample pressures of a few mTorr. The precision describes the uncertainty to which a single frequency measurement can be determined from the TuFIR spectrum. It is a statistical quantity, and is usually expressed as the standard deviation of a spectral point from its mean value, i.e. [3]: 1 n 2 2 i i 1 (3.6) n where n is the number of times each point is recorded, i are the individual frequency measurements, and is the mean frequency measurement, given by: 1 n i n i 1 (3.7) 68 CHAPTER THREE SPECTROMETER DEVELOPMENT The mean frequency is usually quoted to a particular accuracy (). The accuracy of the mean is determined from the statistical and systematic errors in the system, whereas the precision only depends on the statistical errors. In this experiment, the precision is affected by: 1. the accuracy to which the FIR laser frequency is known, 2. frequency drift of the FIR laser during a scan, 3. the ‘rate’ at which data is recorded, 4. the digitisation across the scan range, 5. the number of individual readings used to compute a single point in each spectrum. If the laser system is allowed to warm-up and stabilise for around 2 hours, the frequency drift can be eliminated. In the Cambridge TuFIR spectrometer, a laser line is only used to generate TuFIR sidebands if its frequency has been accurately measured elsewhere [7]. Typically these measurements are accurate to a few hundred kHz [7]. This chapter describes the development work undertaken to improve the overall performance of the Cambridge TuFIR spectrometer. Firstly, the system alignment and signal optimisation are described, with particular reference to the new spectrometer configuration, (B). This chapter then explains in detail, the three major developments that were introduced to the TuFIR spectrometer during this PhD, i.e.: 1. the removal of the FPI, 2. the Brewster window absorption cell, 3. computerised control of the spectrometer. Throughout the chapter, these improvements are illustrated with TuFIR spectra of SO2 and CHF3. All the changes that have been made to this spectrometer by the author are summarised. The sensitivity, resolution, and precision of the final spectrometer configuration are highlighted. 3.1 Alignment of the TuFIR Spectrometer The spectrometer alignment was necessary to optimise the throughput of the TuFIR sidebands, thereby increasing the spectrometer sensitivity. The stability of the laser system also affected the sideband power and noise levels. Consequently, TuFIR spectra could only be recorded when both the laser and optical systems had been aligned. 69 CHAPTER THREE SPECTROMETER DEVELOPMENT 3.1.1 Laser System Alignment The vacuum integrity and intra-cavity alignment of both lasers affected their output power and mode quality. In practice, the system was realigned about once a year. Figure 3.2 shows the optical arrangement used to align the lasers in configuration B. Mirror H is greyed out because it is removed during any stages involving the alignment of the CO2 laser. The He-Ne beams were diffracted as they passed through the FIR laser resonator and the irises A, D, G, and J. The superposition’s of these diffraction patterns were used to judge the alignment accuracy. In addition, a small proportion of each beam was re-reflected from the front face of the ZnSe lens and used to check the lens alignment. In figure 3.2 these patterns are shown at strategic points around the optical system. The alignment procedure for the CO2 laser is outlined in the PL6 instruction manual [8]. The output coupler scatters any incident He-Ne radiation so the whole PZT stack was removed from the laser for alignment purposes. The Brewster windows at the ends of each discharge tube were replaced with PTFE ‘blanks’. A PTFE insert, with a small hole drilled down the middle, was pushed down the full length of each tube to check its vertical and horizontal alignment. Mirror F was adjusted to align the He-Ne down the first discharge tube: the two intra-cavity steering mirrors were used to adjust the beam position in the second tube. The reflection from the diffraction grating was a series of vertical spots that tracked through the iris at G as the grating angle was changed. In practice, the diffraction grating also had to be horizontally adjusted. When the brightest spot was set to pass though iris G it could just be observed as a diffuse image at the output coupler of He-Ne 1. Mirror H was then positioned in front of the CO2 laser and He-Ne 2 aligned through the system as shown in figure 3.2. The input coupler was then repositioned on the FIR laser ensuring that both He-Ne beams passed directly through the 1.5mm hole at the centre of the mirror. The diffraction pattern from He-Ne 1 could no longer be observed at E, but was reflected back by the input coupler mirror to D. When these specular reflections were positioned co-centrically with the other diffraction patterns, the mirror was aligned perpendicularly to the resonator tube. Once the Brewster window had been replaced on the end of the FIR laser, He-Ne 2 and all the optics from F to J were 70 CHAPTER THREE SPECTROMETER DEVELOPMENT . . G CO2 Laser He-Ne 2 F H J . I E He . . Ne 1 . FIR Laser Iris G almost closed A . Iris G fully open D . C B . . Iris D almost closed Iris D fully open Figure 3.2: The optical arrangement used to align the FIR Laser system in Configuration B. The output frequency of both He-Ne lasers is actually red, but HeNe 2 is shown in green here for clarity. See text for details. 71 CHAPTER THREE SPECTROMETER DEVELOPMENT raised by 2mm to retrieve the alignment. After this, He-Ne 2 was no longer required and mirror H was removed. The Brewster windows were then replaced at each end of the CO2 laser discharge tubes and the beam was shifted back down 2mm by the first window and up 2mm by the second. The diffraction grating angle was adjusted to accommodate the beam shift at this point. The output coupler and PZT stack were then repositioned on the CO2 laser. The laser was operated at low powers (<1W) by reducing the gas flow rate (<1mBar residual pressure) and discharge current (10mA). The orientations of the output coupler and PZT stack were iterated until a near field mode pattern resembling TEM00 was observed. The horizontal alignment of the diffraction grating was also changed to compensate for these adjustments at the other end of the laser cavity. A ‘shadow’ was often observed in the near field intensity pattern because some of the radiation transmitted by the output coupler was re-reflected from its back face and was lasing slightly off-axis within the cavity. This secondary image was superimposed on the main mode pattern once the output coupler wedge was orientated perpendicularly to the cavity. The mode quality and low power output of the 9R4, 9P2, 10R4, and 10P2 lines were then tested. A spectrum analyser was used to check the diffraction grating calibration. Once laser action had been re-established in the CO2 laser, the output beam was not usually coincident with He-Ne 1. The height and position of the CO2 laser were adjusted until the radiation was focused right through the FIR input coupler hole. Iris G was fully opened and a power meter was placed consecutively in front of the CO2 laser, and at D. The power at the end of the FIR laser was typically between 70 and 80% of the CO2 laser output power. The throughput could be optimised by making very small adjustments to mirror F or the focusing lens. The power throughput was then re-tested on one laser line in each P- and R- branch. Finally the output coupler was repositioned on the FIR laser. The dichroic mirror was opaque at visible wavelengths, so the incoming He-Ne beam was reflected straight back onto the aperture of the He-Ne laser when the output coupler was aligned. The FIR cavity was pumped for 24 hours then cooled. FIR laser action was re-established on the strongest FIR laser lines, e.g. HCOOH and CH3OH lines. The position of the dichroic output coupler was then manually adjusted to optimise the power output and stability. 72 CHAPTER THREE SPECTROMETER DEVELOPMENT Laser Gas C2H5F Output (GHz) (where (m) known) 1069 Pump Line >5mW output* Sidebands Obtained 9R10 Cu Waveguide Inserted - 660 9R16 577.5511 519.075 9R4 596.8842 502 9R24 - 486 9R24 - 452 9R22 739.3075 405.504 9R30 - 378 10R32 - 376 10R14 - 362.1 10R18 - 336.7 9R16 - 330.2 10R22 - 217.1 10R14 729.9328 410.712 10R12 33mW 846.4503 354.176 10R16 20mW 869.5227 344.778 10R04 12mW 952.2039 314.841 10R10 8mW 1003.5366 298.736 10R24 16mW CH2CF2 375.545 375.5 10P12 17mW CH2CHF 902.0016 332.364 10P6 6mW CH2F2 783.486 382.639 9P10 28mW 887.551 337.97 9R14 6mW 918.4170 326.423 9R14 19mW 1035.5527 289.5 9P4 7mW 1042.1504 287.667 9R34 12mW 1100.8067 272.339 9P10 12mW 1145.4301 261.729 9P38 5mW - 235.5 9R6 15mW CD3OD 73 CHAPTER THREE SPECTROMETER DEVELOPMENT - 227.66 9R18 1397.1186 214.579 9R34 6mW - 196.1 9R32 9mW 1779.1393 166.631 9R20 14mW 1891.2743 158.513 9P10 8mW 2546.495 117.727 9R20 15mW 2742.946 109.290 9P24 CH3Cl 858.0533 349.387 10R18 5mW CH3F 604.2973 496.101 9P20 6mW CH3OD 980.5916 305.726 9R08 26mW CH3OH 525.4275 570.569 9P16 42mW 812.1954 369.114 9P16 1838.8393 163.034 10R38 30mW 252.27816 118.8 9P36 55mW 3105.9368 96.522 9R10 26mW - 326.6 10P6 32mW - 310.8 9P34 16mW - 310 10R34 28mW - 307.5 10R30 25mW 580.5629 516.382 10R08 17mW 790.5046 379.242 10R40 14mW DCOOD 787.7555 380.565 10R12 7mW HCOOD 649.9410 461.261 10P16 6mW 810.3205 369.968 10R28 18mW 716.1565 418.613 9R22 33mW 761.6065 393.631 9R18 37mW CHFCHF COF2 HCOOH * Powers were calculated from the peak-to-peak voltage response of a Golay Detector placed directly in front of the FIR output. Table 3.1: The laser lines observed from the Cambridge FIR Laser system. Those lines with frequencies between 600GHz and 1.2THz are highlighted. A star indicates those lines suitable for TuFIR spectroscopy. (See Ref. [7] for further details of each laser transition). 74 CHAPTER THREE SPECTROMETER DEVELOPMENT The FIR laser was then tested on the weaker laser lines (P<500W) and long wavelength lines (>600-1000m), which were harder to couple out of the laser cavity. Some of the weaker lines were only observed with a 22mm-bore, hollow Cu waveguide inside the FIR cavity. The lines observed on the Cambridge FIR system are summarised in table 3.1. Theoretically TuFIR sidebands can be generated from any FIR laser line, but in this experiment only very stable lines with output powers above 10mW and output frequencies between 600GHz and 1.2THz gave sidebands that were suitable for TuFIR spectroscopy, (table 3.1). The most stable FIR lines are those where no line competition exists between laser transitions that share a common pump line, or those transitions that are uniquely pumped. 3.1.2 Optical System Alignment The optics were aligned in three stages: 1. ‘rough’ system alignment with a He-Ne laser, 2. accurate alignment of the FIR laser signal through the system, 3. detection and optimisation of the TuFIR sideband power at the detector. There was no suitable method for observing the FIR or TuFIR beams directly, so after the ‘rough’ alignment stage the signals were aligned from their peak-to-peak signal intensity, as measured on an SS-5750 Iwatsu oscilloscope [9]. The three optical alignment stages are shown in figures 3.3 to 3.5, (for configuration B). The behaviour of many of the spectrometer optics differed at visible and FIR frequencies, so it was not possible to align the system accurately with the He-Ne laser. Also, the 1/e2 value of the He-Ne beam-waist was only 0.8mm, compared to a waist size of 7mm for the FIR laser beam and 10mm for the TuFIR beam. The grid polariser and the focusing mirrors were replaced with plane mirrors, C, D, and F for the He-Ne alignment. Assuming that the FIR laser system had already been aligned, He-Ne 1 was coincident with the FIR laser output. The height of He-Ne 2 was adjusted accordingly, and both beams were set parallel to the optical bench over a 4m pathlength, (figure 3.3). Mirror C was then placed in front of the FIR laser and He-Ne 2 aligned through the diplexer. This alignment was only approximate as the Melinex transmitted almost all the He-Ne radiation incident on it. The He-Ne beam exited the diplexer at port 2, and was focused onto the apex of the corner cube from the off-axis parabolic mirror (OAP). A 75 CHAPTER THREE SPECTROMETER DEVELOPMENT ‘dentists mirror’ was used to check that the beam was focused to a sharp point at the whisker antenna. pump CO2 Laser ZnSe Lens I He-Ne 2 J He Ne FIR 1 Laser E CO2 laser A He-Ne 1 beam He-Ne 2 C D B H G OAP Diode Diplexer Figure 3.3: The first optical alignment stage. Figure 3.4 shows the FIR alignment. Once FIR laser action had been established, the mirror at C was replaced with a Golay cell and a 15Hz chopper, (to modulate the beam). The Golay detector was linked directly to the oscilloscope to monitor the laser power. A Teflon lens was used to collimate the FIR beam and the laser system adjusted until the Golay’s response was ‘super’-saturated, (figure 3.4a). The Golay cell was then removed and the FIR beam was directed through the diplexer onto the mixer diode: 0.7V of forward bias was applied to the diode, equivalent to 200A current. An oscilloscope was used to monitor the voltage response of the diode to the incident FIR radiation. Initially this signal was less than 2mV. By adjusting the diplexer pathlength difference, OAP mirror and the cube position, the signal could be optimised to around 50mV, 76 CHAPTER THREE SPECTROMETER DEVELOPMENT equivalent to 600A of induced current at the diode. The grid polariser (GP) was then placed in front of the laser at 45o to the incoming radiation. The grid wires were rotated until no signal loss was observed at the diode. A Golay cell was used to check the intensity of the reflected FIR radiation. Typically this was less than 1% of the total beam intensity and was difficult to distinguish from the Golay’s response to the fluorescent lighting. The InSb ‘hot electron’ detector was then placed opposite the Golay cell to detect the FIR radiation that had been re-transmitted from the diode, (figure 3.4b). The detector response was also monitored on the oscilloscope. A Teflon lens, with a 15cm focal length, was used to focus the incoming FIR radiation through the HDPE detector window and into the detector light pipe. Usually, the incoming beam at the diode was not well matched to the antenna angle so the re-radiated FIR beam followed a significantly different path out of the diplexer system. Consequently, the oscilloscope signal from the InSb detector was just visible above the detector noise, (6 to 8mV). The diplexer, OAP mirror, and cube positions were adjusted iteratively until the oscilloscope registered 3 to 4V from the detector and 80mV from the diode, (equivalent to an induced current of 1mA). When irises G and H were removed, the S:N ratios at the detector and the cube were unchanged, provided that there was no leakage from ports 3 and 4 of the diplexer. The two focusing mirrors, D and F, were then aligned. Interesting ‘cavity effects’ were found throughout the spectrometer between consecutive sets of optics. As the position of each mirror was altered, the beam intensity varied sinusoidally. These local ‘maxima’ were used to adjust individual optics and maximise the throughput of the FIR beam. The FIR beam was focused 30cm away from the two mirrors D and F, with a minimum beam diameter of 1cm. Beyond this point the beam intensity dropped off rapidly, i.e. at 1m, the intensity was 1/10th of its original value, and the beam diameter was 2cm. Further investigations showed that this cavity–like behaviour was even more acute if the mirrors at D and F, (with 1m radius of curvature (roc)), were replaced with plane mirrors, or mirrors with shorter focal lengths. Eventually, 2.5m-roc and 4.4m-roc mirrors were used to focus the FIR beam 1.5m from D and F. Minor adjustments were made to all the optics until the detector signal was 3-4V and the diode signal 80mV, (at the oscilloscope). The FPI was then placed close to the focal point of the beam, to 77 CHAPTER THREE SPECTROMETER DEVELOPMENT a. Initial Golay Cell Alignment Oscilloscope FIR Laser Lens Chopper Golay Cell G OAP FIR H Diplexer Diode b. InSb Detector Alignment InSb ‘hot electron’ Detector FIR F Laser FPI Golay Cell D FIR Oscilloscope H GP G OAP Diplexer Diode Figure 3.4: The alignment of the FIR beam through the TuFIR Spectrometer. Optics shaded light grey show their temporary positions during the alignment. 78 CHAPTER THREE SPECTROMETER DEVELOPMENT maximise its throughput, (section 2.3.3). The plate positions were adjusted until discrete transmission peaks were observed. The oscilloscope signal from the detector was reduced to about 70% of its original value. Figure 3.5 shows the set-up used to detect and maximise the TuFIR sidebands. With the chopper removed the maximum current on the diode rose to around 1.5mA. When the microwave source was switched on the diode bias was reduced to around 0.05V, to limit the total diode current to 2mA. The microwaves were AM modulated, with a 1.5kHz square-wave, so the re-radiated TuFIR and FIR signals were also modulated. With the FIR laser switched off, the maximum noise signal at the oscilloscope was 5mV peak-to-peak, corresponding to the electrical and background noise contributions to the detector output. This gave a minimum detectable power of 1.25W. When the signal was filtered and amplified, (60dB), the minimum detectable power was around 10nW, so the InSb detector signal was fed to the oscilloscope via the bandpass filter and amplifier. InSb ‘hot electron’ Detector FPI Preamplifier & bandpass filter FIR TuFIR = FIR -wave F Laser FIR Signal Generator D GP OAP Swept Frequency Microwave Synthesiser -wave GP Diplexer Diode Oscilloscope FIR sideband FIR radiation TuFIR radiation Figure 3.5: The alignment of the TuFIR sidebands through the Spectrometer. 79
