The most important thing is that you determine the input and output signals. In the case of our rare earth doped crystals, the input signal or control signal is a laser. By changing the experimental conditions, such as laser power, temperature, etc. We can just get some output signals. This opens up the possibility of designing devices.

It depends on the materials you use to induce tunable generation.For example, with nonlinear materials, it is possible to achieve tunability by changing the material's refractive index using pumped light; with phase-change materials, tunability can be achieved by temperature changes.