Titanium:Sapphire (Ti:S)-based laser systems have revolutionized ultrafast research from biological imaging to high energy physics. Ti:S has a very broad gain bandwidth (680-1080 nm) but many applications require even broader tuning ranges covering UV, visible, and longer IR wavelengths. Frequency doubling, tripling, and quadrupling extend access to the 190-540 nm range. Ultrafast optical parametric oscillators (OPOs) pumped by Ti:S lasers reach beyond 1080 nm and fill in the “Ti:S gap” (540 to 680 nm).
Now, a new generation of ultrafast OPOs, based on so-called fan-poled crystals, enable breakthrough performance including independent tuning of the Ti:S laser and OPO, higher infrared power, direct access to wavelengths up to 4 μm, and even access up to 20 μm by using difference frequency generation (DFG). Together with its flexible femtosecond and picosecond operation, this new OPO generation greatly extends the utility of Ti:S systems and also enables a single laser resource to be optimized for many diverse experiments.
Ultrafast OPO Basics
An OPO utilizes a non-linear crystal to perform a process called parametric conversion, which is the opposite of sum frequency mixing. In an OPO, an input “pump” photon is used to produce two photons of lower energy, referred to as the signal and idler photons. As shown in Figure 1, the sum of the two photons conserves the original photon energy (frequency). By convention, the higher photon energy (shorter wavelength) output is referred to as the signal beam and the other, longer-wavelength beam is called the idler.
Unlike a laser, the OPO crystal does not store energy as a population inversion; it will only emit light during the time it is pumped, starting from a low efficiency process called parametric spontaneous emission. This has several consequences. First, the output of the OPO replicates the pulse duration of the pump, which can be CW, nanosecond or ultrafast. Second, when used in a high-repetition rate ultrafast system with relatively low energy (a few nanoJoules), at least one of the OPO wavelengths has to be “resonated” in the OPO cavity in order to provide efficient conversion of the pump light. Specifically, the cavity length of the OPO is automatically controlled so that the signal (or sometimes the idler) pulse circulates in the cavity with the same repetition rate as the Ti:S pump laser (typically 76 MHz), and gains energy with every pass through the OPO crystal. This has the added benefit of locking the timing of the pump laser and OPO pulses, which is extremely useful in “two-color” experiments such as Coherent Anti-Stokes Raman Scattering (CARS) or pump and probe spectroscopy.
Three Generations of Synchronous OPO Technology
A practical OPO requires a crystal several millimeters long in order to produce sufficient parametric gain. However, optical dispersion in the crystal means that the phase relationship between the three wavelengths (pump, signal and idler) has a certain periodicity, leading to inefficient or even zero parametric gain over the total crystal length. In birefringent crystals such as LBO (lithium triborate), the phase velocity of light depends on wavelength and polarization; by having different polarizations and an appropriate direction of propagation in the crystal, the three wavelengths can still be “phase-matched.” First generation OPOs were tuned by adjusting the temperature of the crystal to manipulate the signal/pump wavelength combination at which this phase-match occurs.
The advent of these synchronous OPOs was a major advance that really extended the utility of Ti:S based ultrafast laser systems. Unfortunately, their total tuning range was limited to 200-300 nanometers, as fixed by the properties of the available crystals, together with practical limits on the angles at which the crystals could be cut and the usable temperature ranges.
The successful development of periodically poled (PP) crystals supported the creation of a second generation of OPOs relying on quasi-phase matching. Here, a carefully patterned electric field applied to the crystal during its growth creates a striped pattern of forward and reversed crystal domains. By carefully designing the stripe width, it is possible to achieve local phase matching in all the domains for a broad range of signal wavelengths at the same time. The insertion of a dispersive element, like a block of glass, can then be used to restrict the output wavelength to a narrower bandwidth by synchronizing only one signal wavelength with the pump. The OPO output wavelength can be smoothly and simply tuned by varying the length of the OPO cavity. The main advantages of these second generation OPOs are simplified and extended tuning while keeping the pump wavelength fixed, typically at the peak of Ti:S gain, i.e., 830 nm.
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.