Tham khảo tài liệu 'advances in optical and photonic devices 2011 part 14', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 314 Advances in Optical and Photonic Devices Relaxation-oscillation suppression through second-harmonic generation We have seen that a combination of the long upper-state lifetime of Nd-doped laser gain materials coupled with the high-frequency oscillations resulting from rapid power flow from the pump through the parametric process to the signal cavity leads to very weak damping and hence long oscillation times. One potential route to break this cycle is the inclusion of an additional damping mechanism within the system. Such a scheme could be introduced actively by utilising a fast detector to monitor the circulating pump or signal field strength and then somehow instantaneously feed this back onto the loss of the respective cavity. Fig. 15 shows such a technique implemented in the schematic ICOPO depicted in Fig. 2 b . Fig. 15. Potential active scheme to eliminate relaxation-oscillations via an active damping mechanism In this device the small amount of circulating pump field which reflects off of the antireflection coated beamsplitter is directed towards the fast photodetector. The signal from this detector is then amplified and buffered before being fed into a loss mechanism perhaps an acousto- or electro-optic modulator. This loss mechanism modulates the loss of the pump cavity and hence the cavity photon lifetime thus forming a negative feedback loop which preferentially suppresses the high intensity peaks of the oscillations and significantly reduces their damping time. Numerical modelling of this technique indicates that the dynamics of the system are modified to the point at which the oscillation decay time is potentially even shorter than those exhibited by the laser in the absence of the parametric process. In practice though the finite bandwidth of the feedback loop described here would introduce an unacceptable phase lag between the peaks of the oscillations and the modulation of the intracavity loss a problem exacerbated by the high frequency of
