laser frequency doubling
Laser frequency doubling refers to the laser whose wavelength is minimized by fifty percent, as well as the frequency being doubled through the frequency doubling crystal (LBO, BBO). After the crystal doubles the frequency of 1064 nm solid light, it eventually becomes 532 nm green light.
Doubling condition
The problem for frequency doubling is that the crystal can locate an instruction to make sure that the basic frequency laser with frequency f1 and the frequency doubled light with frequency 2 * f1 can have the exact same refractive index (photon momentum conservation), so that suitable gain quality can exist in the crystal size. The laser can continuously convert the energy from the f1 basic frequency to the f1 doubled frequency.
The principle of optical frequency doubling
The principle basis for the frequency doubling of light is the nonlinear result of laser light. The laser light is so intense that it triggers the atomic polarization of the crystalline material, that is, the separation of positive and adverse charge facilities. This splitting up is a vibrant vibration, and the vibration frequency is consistent with the frequency of the laser. The vibration amplitude is connected to the strength of the laser area. Since the laser electromagnetic field intensity and polarization intensity are nonlinear, for second-order nonlinearity, the polarization strength is proportional to the square of the laser’s electric area strength E.
The strength of the essential frequency optical area varies, which can be seen from the trigonometric feature, cosa * cosa = 0.5 * (cos2a + 1). The second-order nonlinearity will generate double-frequency polarized vibration as well as zero-frequency polarized prejudice. This frequency-doubled polarization (resonance of the range between positive and negative charges) will generate or contribute to the passing of frequency-doubled laser light.
Frequency-doubled light problem
This makeover or enhancement of doubled-frequency light requires to meet two conditions:
- The essential frequency light leads the doubled frequency light by 0.75π;
- The stage difference area stays unmodified in the crystal activity area.
The phase difference room remains the same, requiring the material to have the exact same refractive index for both frequencies. Usually, the refractive index of products enhances with light frequency.
BBO nonlinear crystal similar to this can meet the exact same refractive index in a particular instructions. The constant refractive index ensures that the spatial coupling area with a specific size in a particular direction in the crystal is repaired as well as the waveform distinction is secure. There is a particular variance in practice, so the combining length is limited, which is the particular length of the laser crystal.
Category of frequency-doubling crystals.
Ammonium dihydrogen phosphate (ADP), potassium dihydrogen phosphate (KDP), potassium dihydrogen phosphate (DKDP), dihydrogen arsenate crucible (DCDA), as well as various other crystals.
They are a depictive style of crystals that produce dual-frequency and various other nonlinear optical results, are suitable for use in the near-ultraviolet-visible and near-infrared areas, and have a huge damage limit.
Lithium niobate (LN), salt barium niobate, potassium niobate, α-type lithium iodate, and also various other crystals.
The second nonlinear electric polarization coefficient is big, and the refractive index of crystals such as LN and BNN is sensitive to temperature, which is various from the temperature level modification features of the diffusion effect. People can adjust the temperature properly to attain non-critical matching. Ideal for the visible light area and also mid-infrared area (0.4 μ-5μ).
LN is prone to refractive index modification as well as photodamage under light; the damages threshold of BNN is higher than that of LN, yet the strong remedy area is wider, as well as the composition is easy to change, resulting in inadequate optical harmony, and also large crystals with excellent performance are challenging to get; potassium niobate has no strong service In the melting area, it is feasible to acquire huge crystals with uniform optical homes; α lithium iodate is a liquid option growth crystal, which can expand big crystals with excellent optical top quality, and also the damage limit is higher than that of BNN crystals. The drawback is that it has no non-critical matching capability.
Semiconductor crystals
Semiconductor crystals consist of gallium arsenide, gallium arsenide, zinc sulfide, cadmium zinc oxide, selenium, etc. Their quadratic nonlinear electrical polarization coefficients are above those of the very first two crystals and appropriate for broader infrared bands.
However, with the exception of selenium as well as tellurium, the majority of crystals have no dual refraction effect and also can not achieve placement matching.
Borate, barium metaborate (β-BaB2O4), lithium triborate (LiB3O5), etc.
In the 1980s, researchers successfully established barium metaborate and lithium triborate crystals for the first time. As well as the exceptional advantages of large nonlinear optical coefficients and a high laser damage limit. It is an outstanding crystal product for laser frequency conversion, which has caused wonderful consequences worldwide. Ideal for ultraviolet wavelengths, including KBF, etc, even for deep ultraviolet wavelengths. The fundamental requirements for the sum frequency, difference frequency, and also optical parameter oscillation results of nonlinear optical crystals coincide as those of dual-frequency crystals.
