Refuerzo de soportes de hormigón armado

In conclusion to this chapter we have discussed the concept of phase-matching and quasi phase-matching within a nonlinear medium for the purposes of optical parametric generation and oscillation in the cw regime. The principal usages of cw single-frequency OPOs, e.g. spectroscopic applications and LIDAR, require very spectrally stable outputs, and although cw OPOs are inherently single-frequency by nature, the output from these devices suffers detrimentally from mode-hopping, not only due to temperature fluctuations and pressure changes applied to the cavity and thermal expansion of the PPLN, but in the majority due to the movement of the

PPLN temperature (0C) 120 140 160 180 200 Id le r wavel engt h ( µ m) 3.60 3.65 3.70 3.75 3.80 3.85 3.90

184 parametric gain curve as a function of temperature. Therefore, although phase- matching can now be readily achieved in nonlinear materials and with the advent of new technologies, the cw singly-resonant threshold of such OPO devices can be lowered to a point whereby they can be operated in a cw regime without the need for added cavity complexity via pump enhancement or doubly-resonant cavity designs, they still suffer detrimentally from mode-hopping. In order to provide robust and reliable single-frequency output expensive and costly cavity additions have been made in some cases, locking the cavity of the OPO to a reference or even extending the mode-hop free tuning range, but without overcoming mode hopping entirely due to the sensitivity of the movement of the parametric gain curve with fluctuations in PPLN temperature. However we suggest a different approach: by including a second harmonic generating crystal within the OPO cavity mode-hop suppression could occur, proportional in range to the nonlinear second harmonic generation of the fundamental signal mode, allowing the OPO to operate single-frequency both robustly and reliably.

Initial results have shown that the OPO in its various cavity configurations has behaved in terms of temperature, wavelength tuning and threshold as our theoretical model predicts, however, in order to more closely observe the spectral characteristics of the signal wavelength, an output coupler at the signal wavelengths needs to be incorporated into the cavity. This will increase the threshold of the device but will allow us to observe the OPO mode hopping with temperature fluctuations over time. The following chapter will record our observations of OPO mode hopping and will follow our attempts to suppress mode hopping by the incorporation of a nonlinear crystal into the OPO cavity in an attempt to utilize our passive technique for mode hop suppression that was outlined in the design and building of the Nd:YLF master oscillator.

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