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The violet ECDL was locked to the X0g+→B0u+ (15,0)R(94) 130Te2 line at a frequency of 24,373.3 cm-1, as indicated in Figure 6.4. The variations in transmitted intensity of the locked laser were translated into frequency fluctuations using the gradient of the line (change in transmitted intensity per MHz), which was calculated from a spectrum that was recorded immediately prior to the laser-locking experiment. The variation of the laser frequency as a function of time is plotted in Figure 6.7; the sampling interval was 100 ms. It can be seen that the laser frequency fluctuates around a mean value, rarely deviating by more than 30 MHz (i.e. < 1 part in 107); no long term drift in the laser frequency was observed. One outlying data point is apparent in Figure

6. Laser locking to 130Te2 resonance line 6.7, and this may be the result of a mechanical disturbance. The control signal increased gradually throughout the experiment, corresponding to a shortening of the extended cavity, to compensate for an apparent increase in the ambient temperature or pressure. In order to stabilise the laser wavelength over longer time periods, it would be necessary to achieve a greater degree of passive stability by improving the temperature stabilisation and pressure isolation of the laser housing (Talvitie et al. 1997).

Also shown in Figure 6.7 is a histogram of differences from the mean frequency, along with a fitted Gaussian (FWHM = 39.1 MHz). When compared to the typical transition widths at atmospheric pressure, such as that of the 52P1/2→62S1/2 transition of atomic indium shown in Figure 6.4, the fluctuations in the intensity of the locked laser are very small. This set-up is therefore shown to be appropriate to use for frequency stabilisation of diode lasers for use in gas sensing applications at atmospheric pressure.

Figure 6.7. Variation in frequency of the actively locked 410 nm ECDL; the variation was calculated from the fluctuations in the intensity of the laser beam transmitted through the 130_Te

2 absorption cell. Insert: Histogram of deviations

6. Laser locking to 130Te2 resonance line

6.5 Conclusions

It has shown that there are strong 130Te2 absorption lines suitable for use in frequency referencing of laser scans and for active locking of lasers at wavelengths overlapping with the 52P1/2→62S1/2 and 52P3/2→62S1/2 transitions of atomic indium at wavelengths of around 410.2 nm and 451.1 nm respectively. In the former case this is below the spectral range covered by the tellurium atlas (Cariou and Luc 1980), which stops at 417 nm. The 410 nm ECDL was stabilised to one of the Te2 lines and the frequency fluctuations were within the range of 40 MHz, which is negligible compared to typical transition widths at atmospheric pressure. It has been noted that a number of atomic and molecular species have strong electronic transitions in the wavelength range 394 nm to 417 nm, and frequency-referencing and laser- locking techniques based on 130Te2 absorption could therefore find application in gas sensing experiments.

The overall purpose of the present project is the measurement of flame temperatures using diode-laser TLAF. As mentioned at the beginning of this Chapter, the wavelength-locking technique described here could be useful in this context. One way of increasing the temporal resolution of the temperature sensor would be to lock each of the two ECDLs to overlap with the respective indium transitions, and then to switch rapidly between the two beams using an optical chopper. In this way the integrated line strength would no longer be measured but the intensity of the fluorescence at one point on the spectrum is proportional to the integrated line-strength. The ratio of the two fluorescence signal would thus be related to the temperature, although a calibration constant ‘C’ would be required, whose value would depend on

6. Laser locking to 130Te2 resonance line the positions at which the lasers were stabilised. Since the lasers remain at full power and well-overlapped with the indium transitions, this method would maximise the signal-to-noise ratio, which is an important consideration for high-temporal resolution measurements. It is therefore worth pursuing this laser-locking approach as a part of the effort to achieve high-speed TLAF thermometry suitable to dynamic combustion systems.

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6. Laser locking to 130Te2 resonance line

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