laser frequency doubling
Laser frequency doubling describes the laser whose wavelength is reduced by half, as well as the frequency is doubled with the frequency doubling crystal (LBO, BBO nonlinear crystal). After the crystal doubles the frequency of 1064nm strong light, it eventually becomes 532 green light.
Doubling condition
The condition for frequency doubling is that the crystal can locate a direction to ensure that the fundamental frequency laser with frequency f1 and also the frequency doubled light with frequency 2 * f1 can have the same refractive index (photon momentum preservation), to make sure that excellent gain attribute can exist in the crystal length. The laser can constantly transform the power from the f1 essential frequency to the 2 * f1 frequency doubled light.
The concept 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 causes the atomic polarization of the crystalline material, that is, the splitting up of favorable and unfavorable charge facilities. This splitting up is a dynamic vibration, and the vibration frequency is consistent with the frequency of the laser. The resonance amplitude is connected to the intensity of the laser area. Because the laser electromagnetic field strength as well as polarization strength are nonlinear, for second-order nonlinearity, the polarization strength is proportional to the square of the laser’s electrical field intensity E.
The strength of the essential frequency optical area fluctuates, which can be seen from the trigonometric feature, cosa * cosa= 0.5 *( cos2a +1). The second-order nonlinearity will certainly generate double-frequency polarized vibration and zero-frequency polarized bias. This frequency-doubled polarization (resonance of the distance in between favorable and also unfavorable charges) will create frequency-doubled light or contribute in acquiring the passing frequency-doubled laser light.
Frequency-doubled light condition
This transformation or enhancement of doubled-frequency light needs to fulfill 2 problems:
- The essential frequency light leads the doubled frequency light by 0.75 π;.
- The stage difference room continues to be unmodified in the crystal action area.
The stage distinction area stays the same, requiring the material to have the very same refractive index for both frequencies. Usually, the refractive index of materials boosts with light frequency.
BBO crystals such as this can fulfill the same refractive index in a specific instructions. The regular refractive index guarantees that the spatial coupling area with a specific length in a specific direction in the crystal is fixed and the waveform difference is secure. There is a certain variance in practice, so the combining length is limited, 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 various other crystals.
They are a depictive variety of crystals that create dual-frequency and also other nonlinear optical effects, appropriate for use in the near-ultraviolet-visible as well as near-infrared areas, and also have a huge damages threshold.
Lithium niobate (LN), salt barium niobate, potassium niobate, α-type lithium iodate, and various other crystals.
The second nonlinear electrical polarization coefficient is large, and the refractive index of crystals such as LN and also BNN is sensitive to temperature level, which is different from the temperature change qualities of the diffusion effect. Individuals can change the temperature level appropriately to accomplish non-critical matching. Appropriate for the noticeable light region and also mid-infrared region (0.4 μ-5μ). LN is prone to refractive index modification and photodamage under light; the damages threshold of BNN is more than that of LN, yet the solid solution area is larger, as well as the make-up is very easy to alter, resulting in bad optical harmony, as well as large crystals with superb efficiency are difficult to acquire; potassium niobate has no solid option In the melting area, it is feasible to get big crystals with uniform optical homes; α lithium iodate is a liquid solution growth crystal, which can expand big crystals with good optical quality, and also the damages threshold is more than that of BNN crystals. The drawback is that it has no non-critical matching capability.
Semiconductor crystals.
Semiconductor crystals include gallium arsenide, gallium arsenide, zinc sulfide, cadmium zinc oxide, selenium, and so on. Their quadratic nonlinear electrical polarization coefficients are above those of the initial 2 crystals and also are suitable for bigger infrared bands.
However, besides selenium as well as tellurium, many crystals have no dual refraction result and can not accomplish placement matching.
Borate, barium metaborate (β-BaB2O4), lithium triborate (LiB3O5), etc.
Amongst them, Scientists efficiently developed barium metaborate as well as lithium triborate crystals for the very first time in the 1980s. And also had the impressive advantages of large nonlinear optical coefficients and also high laser damages threshold. It is an outstanding crystal product for laser frequency conversion, which has created fantastic repercussions worldwide. Suitable for ultraviolet wavelengths, including KBF, and so on, even for deep ultraviolet wavelengths. The basic needs for the amount frequency, distinction frequency, as well as optical specification oscillation effects of nonlinear optical crystals are the same as those of dual-frequency crystals.
