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.