Acoustic modulator

One method of mode locking a visible laser is by placing an acoustic modulator in the cavity and driving it at a frequency of c/2d. 

Under low-frequency excitation, the flame front is wrinkled by velocity modulations (Fig. 5.2.5). The number of undulations is directly linked to frequency. This is true as far as the frequency remains low (in this experiment, between 30 and 400 Hz). The flame deformation is created by hydrodynamic perturbations initiated at the base of the flame and convected along the front. When the velocity modulation amplitude is low, the undulations are sinusoidal and weakly damped as they proceed to the top of the flame. When the modulation amplitude is augmented, a toroidal vortex is generated at the burner outlet and the flame front rolls over the vortex near the burner base. Consumption is fast enough to suppress further winding by the structure as it is convected away from the outlet. This yields a cusp formed toward burnt gases. This process requires some duration and it is obtained when the flame extends over a sufficient axial distance. If the acoustic modulation level remain low (typically v /v < 20%),... 

A.L. Birbaud, D. Durox, S. Ducruix, and S. Candel. Dynamics of confined premixed flames submitted to upstream acoustic modulations. Proc. Combust. Inst., 31(1) 1257-1265, 2007. 

Moreover, in recent years broad band lasers have appeared which lack any frequency modal structure, at the same time retaining such common properties of lasers as directivity and spatial coherence of the light beam at sufficiently high spectral power density. The advantages of such a laser consist of fairly well defined statistical properties and a low noise level. In particular, the authors of [245] report on a tunable modeless direct current laser with a generation contour width of 12 GHz, and with a spectral power density of 50 /xW/MHz. The constructive interference which produces mode structure in a Fabry-Perot-type resonator is eliminated by phase shift, introduced by an acoustic modulator inserted into the resonator. 

Figures 15.7a and 15.76 show the results of acoustic-modulation and fuel-modulation control, respectively. Controllers used include phase-delay, LQG-LTR, and adaptive posi-cast controllers. With acoustic modulation, phase-delay and LQG-LTR control led to a reduction of about 4 and 26 dB in the amplitude of rms pressure fluctuations. Thus, a significant improvement (22 dB) was achievable with LQG-LTR control. Note that the advantage of acoustic modulation is that a variable gain/amplitude can be dialed into the control signal, and this capability is exploited by the model-based control. The input energy to the loudspeakers with an LTR controller is varied in proportion to the feed-...

Figure 15.7 Pressure spectra with and without control (a) acoustic modulation (J — without control 2 — with LQG-LTR control 3 — with phase-delay control at 0 ms and 4 — with phase-delay control at 3 ms) and (6) fuel modulation [1 — base line 2 — phase delay 3 — LQG-LTR and 4 — adaptive posi-cast). Flow conditions in (a) and (6) correspond to inner and annular airflow rates 16 and 44 cfm. Primary and secondary fuel flow rates 2.0 gph.

Active control studies on a swirl-stabilized spray combustor are presented. Significant improvements with model-based control over traditional time-delay control is demonstrated in the present work. These improvements are particularly noted with acoustic modulation. Future work in this area is directed toward using a proportional drive spray injector where the full amplitude/phase information from the model-based controller can be exploited. 

Nevertheless, the method was realized with radial and tangential director orientation on the cylinder surfaces with acoustical modulation of the polarization signal [195]. The flexoelectric moduli were deduced firom the relations... 

Weight reduction Acoustic modules —Favors lightweight floor acoustics" -On-module acoustics... 

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