The working process of nonlinear crystal BBO (β-barium borate) is mainly based on its optical nonlinear effect, and the following is how it works in different applications:
Laser frequency conversion
- Frequency doubling process: When a strong laser beam (such as the 1064nm infrared laser emitted by an Nd:YAG laser) passes through the BBO crystal, the nonlinear properties of the crystal will cause the laser's electric field to cause polarization within the crystal lattice, resulting in the emission of new photons with twice the frequency of the incident beam, thereby converting the infrared laser into visible light (such as green light at 532nm).
- Triple and quadruple frequency: By adjusting the angle of incidence and polarization state of the laser, BBO crystals can also achieve higher frequency conversions such as triple frequency (such as converting 1064nm laser to 355nm ultraviolet light) and quadruple frequency (such as converting 1064nm laser to 266nm deep ultraviolet light).
Optical Parametric Oscillation (OPO)
- Generating New Photons: During the OPO process, the BBO crystal interacts with the pumped laser beam to convert the energy of the pumped light into two new photons of different frequencies, namely signal light and idle frequency light, through nonlinear optical effects, resulting in amplified and tunable laser output in a specific wavelength range.
- Wavelength tuning: By changing parameters such as the crystal's temperature, angle, or the wavelength of the pumped light, the output laser wavelength can be tuned to produce tunable coherent light over a wide spectral range, from UV to mid-infrared.
Electro-optical modulation
- Controlling the polarization state of light: BBO crystals control the polarization state of light by applying an external electric field to change the refractive index of the crystal in an electro-optical modulator. When the laser beam passes through an electro-optically modulated BBO crystal, its polarization state changes with the change of the applied electric field, thereby modulating the laser intensity, phase, or polarization.
- Application in optical communication and lidar: In optical communication systems, electro-optical modulated BBO crystals can be used to achieve high-speed optical signal modulation and demodulation; In LiDAR systems, it can be used to generate pulsed laser signals and detect echo signals.
Quantum optics
- Generate entangled photon pairs: In quantum optics experiments, BBO crystals can generate entangled photon pairs through a nonlinear optical process. When the laser shines on the BBO crystal, the nonlinear effect of the crystal splits a photon into two entangled photons, which have a specific quantum state association and can be used in research in fields such as quantum communication and quantum computing.