Tham khảo tài liệu 'advances in optical and photonic devices part 17', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Practical Continuous-Wave Intracavity Optical Parametric Oscillators 309 when mirror M2 was replaced with an optimal output coupler for the pump cavity 510mW of power at the pump was obtained. We can therefore take the down-conversion efficiency of the device that is the fraction of the total obtainable power which is down-converted to be 476 510 93 . A down-conversion efficiency of unity can only be achieved when the OPO is optimally output coupling the pump field through the parametric effect which is achieved when relation 1 is satisfied. For a laser threshold and operating pump power of 69 and 1000mW respectively the optimal OPO threshold is then 250mW - slightly less than is the case in this system. As we have said the stability of the cavity has been improved by slightly defocusing the pump and signal waists within the PPLN crystal which has raised the OPO threshold to this non-optimal level. The resulting improvement in performance however makes this slight drop in overall efficiency a price worth paying. Fig. 11. Spontaneous and long-lived bursts of relaxation-oscillations manifesting themselves on the circulating pump field. Finally we turn our attention to the transient stability of the device which was measured by directing the small amount of pumping field reflected off of the rear face of the beamsplitter onto a fast photodetector. An example of the resulting trace is shown in Fig. 11. In the absence of any external perturbation mechanism the pump and hence signal and idler fields exhibited spontaneous and very long-lived bursts of high frequency relaxation oscillations. This resulted in 100 modulation of the extracted idler field and erratic longitudinal mode hopping of the pump field both of which are most undesirable in the context of high resolution spectroscopy and renders the device unsuitable for all but mean power crude mid-IR applications. This is regrettable as in all other respects this system displays the very many highly desirable .
