Optical Parametric Oscillation

In section 6.2.1, we discussed how the signal (and idler (waves may build up from background noise by means of the parametric amplification process. If the nonlinear medium was placed in an optical cavity that was resonant at one of the generated wavelengths, then this would lead to a device with similarities to a laser oscillator. The difference is that while a laser operates by the principles of stimulated emission, the oscillator discussed here relies on parametric processes to provide optical gain. After many passes of the cavity, significant power could be converted from the pump to the generated down-converted wavelengths. This is the basic principle of operation of an optical parametric oscillator (OPO), a basic diagram of which is shown below in figure 6.8.

Figure 6.8. Basic OPO cavity.

Despite enhancements in technology used to implement the technique, the basic methods and design facilitating OPOs are very similar to that outlined by the first paper on the subject by Giordmaine and Miller [12]. Figure 6.8 shows a singly resonant OPO, for which only one of the down-converted waves (in this case the signal wave is resonant in the cavity. The resonant wave can make many trips around the cavity while the in the case of non-resonant waves, only one pass is made.

The singly resonant oscillator (SRO) is the simplest type of OPO. With only one resonant condition to satisfy, the SRO can be tuned continuously, making the device perform consistently as system parameters such as crystal angle (in a birefringent system) or temperature are altered. As we can see from Figure 6.8, only the pump beam is externally fed into the cavity and the signal and idler beams build up from noise. Therefore, because of the weak nature of these two signals and the non resonant nature of the pump beam, a very powerful pump, typically of the order of several Watts, is required to achieve oscillation threshold) in this configuration [13] (although sub-Watt threshold pump levels have been demonstrated [14]).

Alternate OPO cavity configurations exist based on different cavity geometries and resonant characteristics that are able to operate with a far lower threshold, thus requiring a less specialist pumping laser. These different geometries impose additional constraints on the system, but these can have implications on the performance and characteristics of the device. As a general rule, oscillators with more resonating waves exhibit lower thresholds than their simpler counterparts but must consequently pay the price of ever more stringent confinements on the cavity parameters.

6.9 Conclusions

In this chapter I have outlined the underlying physics for nonlinear optical frequency conversion where the origins of the nonlinear polarisation and its implications have been discussed. Equations describing fields generated by this polarisation were shown and solved and the phase relationships between interacting waves as well as methods to control it were outlined. In Chapter 7 I will describe semiconductor-based nonlinear frequency conversion devices that I fabricated for this type of work. Beginning from design considerations, I then proceed to present information on how these devices were fabricated and subsequently evaluated.

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