Future Work

In this section, some brief remarks will be made about a number of possible extensions to this research. These areas include: thermometry in sooting flames, increasing the temporal resolution of the measurements for application to turbulent combustion, and investigation of novel techniques for rapid scanning of the ECDL wavelength.

7.2.1 Measurements in sooting flames

Firstly, the TLAF thermometry technique has the potential to be implemented in sooting flames, in which many other laser diagnostics are subject to interferences. In fact, the TLAF technique is especially well-suited to implementation in sooting flames since the consumption of indium atoms to form InOH occurs at a slower rate in fuel rich flames than in lean flames. Studies of sooting flames are often conducted in low pressure burners and such an environment would also be of interest, due to the shallower gradient at the flame front, which could potentially be resolved with TLAF due to the small measurement volume. For implementation in sooting flames a two- detector scheme would be required, since resonance fluorescence is unsuitable in particle-laden systems due to strong light scattering. Since the line-shape temperature measurement could also be implemented in sooting flames, a comparison of the two approaches could be made.

7.2.2 Measurements in turbulent flames

It was shown in Chapter 4 that the fluorescence signal levels obtained are sufficient to perform temperature measurements with a temporal resolution of

7. Conclusions and Future Work

10 kHz, which would allow the study of turbulent flames. The rapid TLAF measurements would be performed by actively locking the laser wavelengths, as described in Chapter 6, and switching between the two beams using a mechanical chopper. This approach represents a convenient strategy for obtaining high-repetition rate measurements, since the excitation sources are continuous-wave. These would provide an insight into the dynamics of temperature fluctuations in turbulent combustion.

7.2.3 Rapid modulation of ECDL wavelength

A strategy has been developed during the present research for tuning the wavelength of ECDLs much faster than is usually possible (Hult et al. 2005). These results have not been presented in this thesis due to the limitations of space, but they represent another exciting aspect of the project. The restriction imposed on the tuning rate by the need for mechanical movement of the grating is obviated here by tuning only the diode laser injection current. This causes the laser to mode-jump between different extended cavity modes. At times the wavelength jumps to a mode that is off-resonance with the indium transition, subsequently returning to resonance after further mode- hops. This leads to a spectrum comprised of peaks that are separated by the free-spectral range of the extended cavity as shown in Figure 7.1. The spectrum shown here was recorded at a scanning frequency of 10 kHz. The LIF spectrum is obtained from the raw data by extracting the locations of the peaks in signal intensity. Also shown in Figure 7.1 is a theoretical spectrum; it can be seen that the peak positions in the experimental spectrum lie close to the theoretical spectrum.

7. Conclusions and Future Work

Figure 7.1. A fluorescence spectrum of the 52_P

1/2→62S1/2 transition of atomic

indium recorded in a time of 100 µs by tuning only the laser current. The darker line is the experimental data. The peak positions on this trace are extracted to form the LIF spectrum. A theoretical spectrum is also shown.

This novel rapid tuning approach opens possibilities for implementation of flame temperature measurements by either TLAF or by the line-shape technique, and could also find application in other high-speed spectroscopy experiments.

References

Hult, J., I. S. Burns and C. F. Kaminski (2005). "High repetition-rate wavelength tuning of an extended cavity diode laser for gas phase sensing." Applied Physics B 81: 757.

Appendix A

Calculation of the optimal theoretical

ratio of piezo-actuator extensions for

single-mode wavelength tuning

As was noted in Chapter 2, it is possible to calculate theoretically the ratio of the extensions that must be generated in each of the piezo-actuators in the grating mount to achieve optimal conditions for mode-hop free tuning. The derivation of this relationship is presented in this Appendix.

Figure A1 shows a plan view of the extended-cavity diode laser (ECDL) configuration. At the right hand side of the main figure, there are two piezo- actuators one above the other which are always moved synchronously and are collectively denoted ‘B’ as was represented in Figure 2.4.

Appendix A A l1 = 45mm l3 = 20mm l2 = 20.5mm Collimating Lens Laser beam B 28.6mm Piezoelectric crystals

Grating (1800 lines per mm)

~0.7mm ~63nm Diode Crystal (Vertical dimension = ~2µm) lC=25mm Data:

Grating groove spacing: d = 556 nm Grating angles: 410nm laser θ = 21.7º 451nm laser θ = 24.0º Laser beam Grating θ Grating Angle

Figure A1. Configuration of the external cavity laser with indication of important dimensions used in calculations.

The objective is to calculate the ratio between the length change of the piezo-actuator denoted ‘A’ to that of piezo-actuators ‘B’ to obtain the correct proportion between translation and rotation. This allows the tuning of the grating feedback profile to be matched with the tuning of the extended cavity mode structure. If we consider applying a voltage ramp to piezo-actuator ‘A’ only, then grating rotation by angle ∆θ

Appendix A

will result, as shown in Figure A2. The rotation will be about a pivotal axis defined by the two piezo-actuators at B. The translation of the grating is performed by an additional voltage modulation applied to all three piezo-actuators synchronously (for ‘A’ this is simply added to the “pivot” signal). The result is a change in the cavity length of the laser.

Laser beam ∆θ ∆θ 2 2 l +l 2 3 l1 ∆p A B

Figure A2. Rotation of the grating. Let ∆p be that part of ∆A which causes the angle change:

∆p = ∆A - ∆B (A1)

The positive direction of ∆p is considered to be downward as indicated in Fig A2. Note that for the angle change shown in Figure A2, ∆θ is negative.

tan(∆θ) = -∆p / l1 (A2)