OPTICAL   PARAMETRIC   OSCILLATORS

      In the process of harmonic generation two photons combine their energy into a single photon. We have also seen that the reverse process, in which a single photon splits its energy into two photons which will have lower energy than the incident photon, is also possible. By conservation of energy, the energy of the resulting photons must sum to the energy of the incident photon. In an optical parametric oscillator (OPO) configuration a pump beam incident on a nonlinear crystal produces two resultant photons at different wavelengths. One wavelength, called the signal, exits the device as the output beam; the second wavelength, essentially useless, called the idler beam, stays within the cavity of the device, as depicted in Figure 1.1. The frequencies of the idler and output beam sum to the frequency of the pump beam (as required for conservation of energy). An OPO is not a laser since it does not amplify; rather, it is an oscillator only. It is, however, a coherent oscillator producing laser light.

Figure 1.1. Simple OPO device.

        The question now arises: If two output beams are produced by the crystal, why is only one used as an output? The answer lies in phase matching. Only one wavelength is phase-matched at a time. This is also how the OPO is tuned: by using the same methods as those used to phase-match a second-harmonic generator crystal. The temperature of the crystal or the angle of the crystal within the cavity can be changed to tune the laser. In the case of angular tuning, often two crystals are used that rotate in opposite directions. This scheme eliminates displacement of the beam in the cavity, which leads to walk-off loss. A common arrangement is to use an Nd: YAG laser to pump an OPO employing either lithium niobate (LiNbO₃) or beta-barium borate (BBO). Using lithium niobate, for example, an OPO is tunable from about 1.4 μm to almost 4 μm by varying its orientation in the cavity from about 44.5 degrees to 51 degrees. Temperature tuning is also frequently used to simplify the mechanical arrangement of the optics.