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Pump-enhanced OPOs
The
implementation of pump-enhancement in the SRO or DRO configuration can reduce
the pump threshold
by over an order of magnitude to the mW level. As a result, pump-enhanced
OPOs can form the basis for compact, widely-tunable parametric sources.
Many of our pump-enhanced OPOs are constructed as a split cavity. This design, shown in the diagram below, has been used by the group to develop pump-enhanced OPOs capable of extended mode-hop-free tuning (see publications section). In order for the resonated pump light to maintain resonance, a suitable frequency locking scheme is required. As the pump is "always on" this type of device presents a locking error signal before the OPO reaches threshold, a significant advantage for this device. Pump sources are required to be single-frequency for pump-enhanced OPOs.
| Split Cavity, Pump-Enhanced SRO. large image |
Pump light (light blue) enters from the right, via the input-coupling mirror, and passes through the nonlinear medium, generating the signal (green) and idler (violet) waves in the process. A beamsplitter, coated to be highly reflecting at the signal and highly transmitting at the pump, creates separate cavities for the resonant pump and signal waves. The non-resonant idler exits the cavity after a single pass. The split cavity design allows the pump and signal cavities to be adjusted independently, removing one of the main causes of complex tuning behaviour in the standard pump-enhanced configuration. Also, intra-cavity elements such as etalons and modulators can be placed in the signal cavity without perturbing the pump. This latter advantage has proved particularly useful in several of our past and current projects.
 |
| Ring cavity PE-OPO (click for larger image). |
We are also interested in implementing the pump-enhanced configuration for OPOs based on some of the newer nonlinear crystals such as RTA and KTP. Whilst the more commonly used lithium niobate (LN) has a high nonlinear coefficient and hence low pump thresholds, these newer crystals have several important advantages. RTA, in particular, offers a much wider transparency range and does not suffer from the thermal effects that can cause serious problems in LN based OPOs. However, due to a low nonlinear coefficient, the high pump thresholds associated with RTA make implementing a pump-enhanced scheme considerably more difficult. As a result, research into pump-enhanced OPOs based on crystals like RTA is still in the early stages.
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