Novel Ultrafast OPO Sources

Tuesday, 7 March 2006

The technique of quasi-phasematching allows nonlinear crystals to be made whose phasematching properties vary spatially, both across their apertures and along their length.

EPSRC project GR/N08735 aims to evaluate novel crystal designs and show that they can be used in a practical way to realise optical parametric oscillators (OPOs) which have high gain and pumping thresholds low enough to be compatible with diode-based femtosecond lasers.

Key approaches include:

(1)using aperiodically-poled (or “chirped”) crystals to artificially enhance the crystal bandwidth, making it possible to use longer crystals which offer higher gain
(2)"photon recycling” in which a double crystal containing two series sections with different phasematching properties recycles the redundant signal wave to increase the idler output of the OPO.

Quasi-phasematching is understood as follows:

If 3 waves exchange energy through c(2) nonlinear coupling in a nonlinear crystal, the maximum conversion occurs after one “coherence length”, π/Dk, where Dk is the difference in the wavenumbers of the 3 waves, Dk=k3-k2-k1.

After this length back-conversion occurs and the process repeats periodically.

Flipping the crystal domain orientation after one coherence length reverses the conversion direction and, instead of back-conversion, another forward conversion step occurs.

Domain inversion is achieved by applying a high voltage across the crystal using a patterned electrode - “periodic poling”. The domain period determines which wavelengths are phasematched.

We have developed a technique called chirped-pulse frequency conversion to improve the broadband gain of femtosecond OPOs. In a fs OPO the finite bandwidth of the pump pulses limits the crystal size which can be used because, with conventional phasematching, the crystal's bandwidth depends inversely on its length.

Femtosecond pulses have broad wavelength bandwidths and consequently, only short crystals may be used in femtosecond OPOs because those of excessive length show strong back-conversion and low gain.

Short crystals mean high thresholds, but it should be possible to reduce threshold values if the crystal bandwidth can be maintained for longer crystals.

Chirping the crystal poling period causes different pump wavelengths to convert at different positions through the crystal. The figure above illustrates how a suitably chirped pump pulse is always phasematched with the same signal wavelength at any point in the crystal. The chirped-pulse/chirped-crystal approach reduces the average pump-power threshold to less than 15 mW, pump depletions of up to 85% (opposite) and pump to signal extraction efficiencies of better than 35%.

Another method, know as photon recycling, allows improved generation efficiency in the mid-infrared outputs of an OPO. In a conventional OPO, each converted pump photon generates one signal and one idler photon, so the ultimate efficiency of an OPO is dictated by the ratio of the photon energies of the pump and extracted output.

When the idler is resonated, the wasted signal power can be recycled to pump a second crystal, also phasematched for the same idler wavelength, giving up to 2 idler photons for each incident pump photon.

The images below illustrate the difference between a conventional OPO and a photon-recycling (or tandem) OPO.

Conventional idler-resonant OPO

Photon recycling idler-resonant OPO

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