Practical Continuous-Wave Intracavity Optical Parametric Oscillators

Abstract

Computer data. In this chapter we have examined various aspects of the cw-ICOPO in the context of realising low threshold, high efficiency and broadly tunable sources of mid-infrared radiation. We have seen that through an understanding of their origin, steps can be taken in order to either manage or eliminate the relaxation oscillations which have to date barred the cw-ICOPO from use in many of the applications to which it is otherwise ideally suited. Due to the constraints of space we have unfortunately had to neglect many of the other design aspects of these devices; most notably spectral line narrowing, mode-hop free tuning mechanisms and output-coupling of the resonant down-converted field. It is hoped that armed with the knowledge provided in this chapter, the reader will have the necessary expertise and confidence to design and construct a cw-ICOPO and then be able to use well established laser frequency control and output coupling techniques, which are in the main entirely portable from their use in the parent pump laser to the resonant down-converted wave, in order to adapt their system to meet his or her precise needs. Neither have we explored the operation of the ICOPO in the Q-switched, pulsed regime: yet another solution to the problem of relaxation oscillations. Here, the long upper-state (storage) lifetime of Nd gain media is turned to our advantage. Whilst the resulting very high intensity peak powers present within the pumping cavity allow the ICOPO to operate over a wider portion of its phase-matching bandwidth, and therefore increases the linewidth of the down converted waves, the system becomes very much less susceptible of the effects of pump and, in particular, signal cavity round-trip loss. This significantly relaxes the tolerance placed upon cavity finesse and therefore allows the tuning bandwidth of the device to be substantially expanded. Through careful gating of the pump cavity Q and taking advantage of high efficiency cavity-dumping of the pump field into signal and idler through the parametric process (Debuisschert, Raffy et. al. 1996), very high repetition rates (100's kHz) are achievable with mean-power efficiencies significantly higher than those normally associated with Q-switched devices (Stothard, Rae et. al. 2009). The system we considered in section 5.2 is a particularly exciting stepping stone on our road to realising a truly utile cw-ICOPO ideally suited to high-resolution spectroscopic applications. As we saw, the use of state-of-the-art, semiconductor-based laser gain media eliminates the problems associated with relaxation oscillations, therefore releasing the potential of the cw-ICOPO. By utilising enhanced, ring-resonator designs coupled with existing well-established tuning mechanisms (such as cavity length control, the use of frequency-selective elements, etc.), very broad tunability with single-frequency, mode-hop free tuning over 100's GHz becomes possible. That the gain bandwidth of VECSEL gain media is dictated by its internal structure, rather than a particular electronic transition, opens up the particularly exciting possibility of realising cw devices capable of penetrating much further into the infrared spectral region. This band is currently inaccessible (to cw.