Source: http://aoot.osa.org/oe/abstract.cfm?uri=oe-27-6-8777
Timestamp: 2019-04-20 04:15:31+00:00

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We have recently experimentally demonstrated that a novel liquid crystal-based photonic transducer for sensing systems could be utilized as an active Q-switch in a miniaturised and integrated waveguide laser system. In this paper, we now present a comprehensive numerical modelling study of this novel laser architecture by deriving a set of equations that accurately describe the temporal optical response of the liquid crystal cell as a function of applied voltage and by combining this theoretical model with laser-rate equations. We validate the accuracy of this model by comparing the results with previously obtained data and find them in excellent agreement. This enables us to predict that under realistic conditions and moderate pump power levels of 500 mW, the laser system should be capable of generating peak power levels in excess of 1.1 kW with pulse widths of about 20 ns, corresponding to pulse energies > 20 μJ. We believe that such a low-cost and ultra-compact laser source could find applications ranging from trace gas sensing and LIDAR to material processing.
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Fig. 1: (a) The liquid crystal cell used in experiments, width=length=2 cm, thickness=4 mm. (b) The layered structure of the active area in a liquid crystal cell.
Fig. 3: (a) The crossed reflectance as a function of half-wave and quarter-wave plate angles and (b) crossed reflectance as a function of half-wave plate angle without (grey) / with a quarter-wave plate rotated by θwpq = 45° (red) for the 9.0 μm thick liquid crystal cell.
Fig. 4: (a) Schematic of the controlled optical loss measurement and (b) under the square wave signals with the constant amplitude of 60V, modulation depths are 30% at 198 kHz, 18% at 326 kHz and 6% at 914 kHz.
Fig. 5: (a) The square wave signals with the frequency of 5 Hz and the duty cycle of 50% are applied on the liquid crystal cell and (b) corresponding optical loss controlled by the electrical signals, in which the modulation depth is around 97% (close to 100%).
Fig. 6: (a) Schematic of the laser setup. In this setup, the pump light is coupled to a waveguide through a dichroic in-coupling mirror. Additionally, the polarizing beam splitter combined with waveplates and liquid crystal cell acts as a actively-controlled variable output-coupling mirror. (b) Cross-section of the depressed-cladding waveguides.
Fig. 7: The crossed reflectance of the 3.3 μm (a) (b) and the 9.0 μm (c) (d) thick liquid crystal cells as a function of time under varying electric signals. The grey line represents experimental measurement results, while the red line represents the corresponding numerical simulation results.
Fig. 8: (a) Experimental (red circles) and simulation results (grey squares) of pulse width and (b) peak power as a function of the applied voltage for the 9.0μm thick cell.
Fig. 9: (a) Experimental results (red squares) and simulation results (grey squares) of average output power as a function of absorbed pump power and (b) simulation results of pulse width and peak power as a function of repetition rate.
Fig. 10: (a) Q-Switched laser pulses with a repetition rate of 5 kHz from the simulation model and (b) the shortest Q-Switched laser pulses from the model with a pulse width of 20 ns and a peak power of 650.94 W. It could be achieved, when the amplitude of applied voltage is 88.05 V, the pump power is 400 mW and the optical losses is reduced to 10.00%.

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