laser frequency doubling


laser frequency doubling

Laser frequency doubling describes the laser whose wavelength is reduced by fifty percent, and the frequency is doubled through the frequency doubling crystal (LBO nonlinear crystal, BBO). The frequency of 1064nm powerful light is doubled by the crystal, resulting in 532 green light.

Doubling condition

The condition for frequency doubling is that the crystal can find a direction so that the basic frequency laser with frequency f1 and also the frequency doubled light with frequency 2 * f1 can have the very same refractive index (photon momentum conservation), to make sure that the ideal gain feature can exist in the crystal length. The laser can convert energy from the f1 essential frequency to the f1 doubled light indefinitely.

The concept of optical frequency doubling

The nonlinear impact of laser light is the basis for the concept of the frequency doubling of light. The laser light is so intense that it creates the atomic polarization of the crystalline material, that is, the separation of favorable and adverse cost centers. This splitting up is a dynamic resonance, and the resonance frequency follows the frequency of the laser. The vibration amplitude is associated with the intensity of the laser area. Because the laser magnetic field intensity and polarization strength are nonlinear, for second-order nonlinearity, the polarization intensity is proportional to the square of the laser’s electric area strength E.

The intensity of the basic frequency optical field rises and falls, as can be seen from the trigonometric function, cosa * cosa = 0.5 * (ccos2a + 1). Second-order nonlinearity produces double-frequency polarized resonance as well as zero-frequency polarized bias. This frequency-doubled polarization (vibration of the range between favorable and adverse charges) will undoubtedly generate frequency-doubled light or contribute to the passing of frequency-doubled laser light.

Frequency-doubled light problem

This makeover or improvement of doubled-frequency light needs to meet two conditions:

  • The fundamental frequency light is ahead of the doubled frequency light by 0.75 π;
  • The stage distinction area remains unmodified in the crystal action area.

The stage difference room remains constant, requiring the product to have the same refractive index for both frequencies. Usually, the refractive index of materials increases with light frequency.

BBO nonlinear crystals like this can have the exact same refractive index in a certain direction. The regular refractive index ensures that the spatial combining area with a certain length in a specific direction in the crystal is dealt with and that the waveform distinction is stable. There is a specific inconsistency in practice, so the coupling size is restricted. This is the characteristic length of the laser crystal.

Classification 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 type of crystal that produces dual-frequency and other nonlinear optical effects, is suitable for use in the near-ultraviolet, visible, and near-infrared regions, and has a high damage 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, as is the refractive index of crystals such as LN and BNN, which is sensitive to temperature, which is evident from the temperature-level change qualities of the dispersion result. Individuals can change the temperature appropriately to accomplish non-critical matching. Suitable for both visible light and mid-infrared wavelengths (0.4 -5).

LN is prone to refractive index adjustment as well as photodamage under light; the damages threshold of BNN is greater than that of LN, yet the strong option area is broader, as well as the structure is very easy to transform, causing poor optical uniformity, as well as huge crystals with superb performance are challenging to acquire; potassium niobate has no solid option In the melting zone, it is possible to obtain large crystals with consistent optical homes; α lithium iodate is a liquid option development crystal, which can grow huge crystals with good optical quality, as well as the damages limit is higher than that of BNN crystals. The downside is that it has no non-critical matching capacity.

Semiconductor crystals

Semiconductor crystals consist of gallium arsenide, gallium arsenide, zinc sulfide, cadmium zinc oxide, selenium, etc. Their square nonlinear electrical polarization coefficients are greater than those of the initial 2 crystals and appropriate for bigger infrared bands.

However, except for selenium and tellurium, a lot of crystals have no double refraction result as well as can not accomplish position matching.

Borate, barium metaborate (β-BaB2O4), lithium triborate (LiB3O5), etc.

Among them, scientists effectively created barium metaborate and lithium triborate crystals for the very first time in the 1980s. And it also had the outstanding benefits of huge nonlinear optical coefficients as well as a high laser damage threshold. It is an outstanding crystal material for laser frequency conversion, which has created terrific consequences worldwide. Appropriate for ultraviolet wavelengths, consisting of KBF and so on, and also for deep ultraviolet wavelengths The basic needs for the sum frequency, difference frequency, and optical criterion oscillation impacts of nonlinear optical crystals are the same as those of dual-frequency crystals.

Read more: Nd:YAG Laser Crystals


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