Laser frequency doubling refers to doubling the frequency with frequency-doubling crystals (LBO, BBO). After the crystal doubles the frequency of 1064nm solid light, it gets 532 green light.
Doubling problem
The condition for frequency doubling is that the crystal can find a direction so that the basic frequency laser with frequency f1 and the frequency doubled light with frequency 2 * f1 can have the same refractive index (photon energy preservation) so that excellent gain characteristic can exist in the crystal size. The laser can continually convert the power from the f1 basic frequency to the 2 * f1 frequency doubled light.
The theory of optical frequency doubling
The principle basis for the frequency doubling of light is the nonlinear impact of laser light. The laser light is so intense that it triggers the atomic polarization of the crystalline material. That is the separation of favorable and unfavorable charge centers. This separation is an emotional resonance, and the resonance frequency is consistent with the frequency of the laser. The vibration amplitude is associated with the intensity of the laser area. Because the laser magnetic field strength and polarization intensity are nonlinear, for second-order nonlinearity, the polarization intensity is symmetrical to the square of the laser’s electric area strength E.
The intensity of the fundamental frequency optical area fluctuates. This can be seen from the trigonometric function, cosa * cosa= 0.5 *( cos2a +1). The second-order nonlinearity will generate double-frequency polarized resonance and also zero-frequency polarized predisposition. This frequency-doubled polarization (resonance of the distance between positive and adverse charges) will generate frequency-doubled light or play a role in obtaining the passing frequency-doubled laser light.
Frequency-doubled light condition.
This makeover or enhancement of doubled-frequency light needs to fulfill 2 conditions:
(1) The fundamental frequency light leads the doubled frequency light by 0.75 π;
(2) The stage difference room remains unchanged in the crystal activity area.
The stage difference area remains the same, requiring the material to have the same refractive index for both frequencies. Typically, the refractive index of products increases with light frequency.
BBO crystals similar to this can meet the same refractive index in a certain direction. The regular refractive index ensures that the spatial combining area with a particular size in a particular direction in the crystal is dealt with. Also, the waveform distinction is secure. There is a specific variance in practice, so the coupling length is restricted, which is the particular size 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), and other crystals.
They are a depictive form of crystals that create dual-frequency and various other nonlinear optical results. They are appropriate for usage in near-ultraviolet-visible and near-infrared areas and have a huge damage threshold.
Lithium niobate (LN), sodium barium niobate, potassium niobate, α-type lithium iodate, and various other crystals.
The additional nonlinear electrical polarization coefficient is huge, and the refractive index of crystals such as LN and BNN is sensitive to temperature level, which is different from the temperature modification characteristics of the dispersion result. Individuals can change the temperature properly to attain non-critical matching.
Appropriate for the visible light and the mid-infrared region (0.4 μ-5μ). LN is prone to refractive index change and photodamage under the light. The damage threshold of BNN is greater than that of LN. However, the strong option area is bigger, and the composition is easy to change, causing bad optical harmony, and huge crystals with outstanding performance are challenging to get. potassium niobate has no reliable service In the melting zone. it is feasible to obtain huge crystals with consistent optical properties; α lithium iodate is a liquid service development crystal. which can grow huge crystals with great optical top quality, and also, the damage threshold is greater than that of BNN crystals. The disadvantage is that it has no non-critical matching ability.
Semiconductor crystals.
Semiconductor crystals include gallium arsenide, zinc sulfide, cadmium zinc oxide, selenium, etc. Their square nonlinear electrical polarization coefficients are greater than those of the first 2 crystals and appropriate for wider infrared bands.
However, except for selenium and tellurium, many crystals have no dual refraction effect and can not achieve position matching.
Borate, barium metaborate (β-BaB2O4), lithium triborate (LiB3O5), and so on.
Amongst them, Scientists successfully developed barium metaborate and lithium triborate crystals for the first time in the 1980s. And had the impressive benefits of large nonlinear optical coefficients and high laser damage limit. It is an exceptional crystal product for laser frequency conversion, which has caused excellent consequences worldwide. Appropriate for ultraviolet wavelengths, including KBF, etc., even for deep ultraviolet wavelengths. The basic requirements for the amount frequency, difference frequency, and optical criterion oscillation results of nonlinear optical crystals coincide with those of dual-frequency crystals.
