PP crystals marked a significant improvement in ultrafast OPOs, however they still constrained the pump wavelength to be kept at a certain fixed value; paradoxically, the tunable Ti:S laser oscillator is used to pump a tunable OPO, yet the Ti:S laser wavelength is more or less fixed when the OPO is in use. For experiments requiring two independently tunable wavelengths, like many pump and probe applications, a second Ti:S laser would have to be used and synchronized to the first Ti:S/OPO setup. This approach is, however, more costly and complicated.
These limitations were finally overcome with the advent of third generation synchronous OPOs based on so-called fan-poled crystals. As illustrated schematically in Figure 2, in these crystals the induced poling follows a variable (fan-shaped) pattern instead of uniform stripes. If the crystal is translated perpendicularly to the beam path, the beam passes through regions with different periodicity enabling the phase matching condition to be smoothly adjusted for different pump wavelengths.
As with conventional PP materials, the signal wavelength is selected by adjusting the OPO cavity length to match the synchronization of the desired signal wavelength to the pump wavelength. With this approach, the Ti:S OPO setup finally delivers on its full potential and value, providing independent and broader tuning of the pump, signal and, if required, idler wavelength (Figure 3).
Fan-Poled OPO Advantages
With fan poling, the Ti:S laser and OPO wavelengths can be separately tuned, thus providing two fully independent wavelengths, if only part of the Ti:S laser intensity is used to pump the OPO. In addition, users requiring mid-IR wavelengths up to 4 μm may optionally use the idler beam transmitted through an independent output port of the OPO. The OPO can also be converted into a ring configuration including an intracavity frequency-doubling crystal to generate 505-750 nm (covering the “Ti:S gap”) with conversion efficiency close to 100%. Finally, when equipped with an optional automated frequency conversion module, a tunable Ti:S plus fan-poled OPO combination provides gap-free tuning to wavelengths as short as 190 nm.
There are many types of ultrafast experiments that can benefit from this independent tuning. In biology for instance, having broad wavelength tuning together with two independently variable wavelengths enables the same compact laser setup to be used for comprehensive multimodal microscopy studies, using all the different types of nonlinear imaging techniques, including CARS, SHG/THG imaging, and multiphoton excitation (MPE). And, in the field of molecular reaction dynamics, studying detailed photochemistry typically requires the ability to independently tune the pump and probe wavelengths.
Fan-poled OPOs can reach much further into the IR than second-generation OPOs, particularly those based on blue pumping with a frequency-doubled Ti:S. For example, a Mira fan-poled OPO can reach 4 μm with the idler output, far beyond the reach of other commercial ultrafast OPOs. Fan-poled technology also enables more efficient difference frequency generation (DFG) where the signal and idler outputs are combined to produce tunable infrared output to wavelengths as long as 20 μm. That’s because for any desired DFG wavelength, the user can select the Ti:S pump wavelength providing the signal/idler combination with the highest combined power and therefore the highest DFG conversion. The flexibility of a fan-poled OPO allows the user to always be able to choose this optimum combination.
Finally, third generation OPOs provide pulsewidth flexibility. So with a typical Ti:S pulse duration of 120 fs, a fan-poled OPO will deliver 200 fs pulses; but these can be shortened to only 80 fs by optionally operating the OPO with a wider output bandwidth. Alternatively, the OPO can be configured for picosecond (2-5 ps) operation in order to get a narrow (e.g., 2 nm) bandwidth for time-resolved studies with higher spectral resolution.
In summary, this new generation of ultrafast OPOs greatly extend the operating wavelength range of Ti:S lasers providing combined spectral coverage from 190 nm to 20 micron, flexible pulse duration and high power output. Independent pump and OPO tuning opens the door to simplified and less costly set-ups for pump and probe and non-linear microscopy applications.