## laser frequency doubling

Laser frequency doubling refers to the laser whose wavelength is minimized by fifty percent, as well as the frequency is doubled through the frequency doubling crystal (LBO, BBO). After the crystal doubles the frequency of 1064nm strong light, it gets 532 green light.

## Doubling problem

The problem for frequency doubling is that the crystal can locate an instructions to make sure that the basic frequency laser with frequency f1 and also the frequency doubled light with frequency 2 * f1 can have the same refractive index (photon energy conservation), to ensure that excellent gain characteristic can exist in the crystal length. The laser can continually transform the energy from the f1 basic frequency to the 2 * f1 frequency doubled light.

## The theory of optical frequency doubling

The concept basis for the frequency doubling of light is the nonlinear effect of laser light. The laser light is so extreme that it creates the atomic polarization of the crystalline product, that is, the separation of favorable and adverse fee centers. This splitting up is a vibrant resonance, and also the resonance frequency is consistent with the frequency of the laser. The resonance amplitude is related to the strength of the laser field. Due to the fact that the laser electromagnetic field strength as well as polarization intensity are nonlinear, for second-order nonlinearity, the polarization intensity is proportional to the square of the laser’s electric field intensity E.

The intensity of the fundamental frequency optical field varies, which can be seen from the trigonometric function, cosa * cosa= 0.5 *( cos2a +1). The second-order nonlinearity will create double-frequency polarized resonance as well as zero-frequency polarized predisposition. This frequency-doubled polarization (vibration of the distance in between favorable and negative fees) will create frequency-doubled light or contribute in obtaining the passing frequency-doubled laser light.

### Frequency-doubled light condition.

This makeover or enhancement of doubled-frequency light needs to fulfill two 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 activity region

The stage difference space stays the same, calling for the product to have the very same refractive index for both frequencies. Usually, the refractive index of materials enhances with light frequency.

BBO crystals such as this can satisfy the very same refractive index in a specific direction. The consistent refractive index makes sure that the spatial coupling location with a specific length in a specific direction in the crystal is dealt with as well as the waveform distinction is steady. There is a particular discrepancy in practice, so the coupling length is restricted, which is the particular length of the laser crystal.

## Category of frequency doubling crystals

### Frequency doubling crystals: ADP, KDP, DKDP, DCDA, etc.

Ammonium dihydrogen phosphate (ADP), potassium dihydrogen phosphate (KDP), potassium dihydrogen phosphate (DKDP), dihydrogen arsenate crucible (DCDA), and also various other crystals. They are a representative kind of crystals that create dual-frequency as well as other nonlinear optical effects, are suitable for usage in the near-ultraviolet-visible and near-infrared areas, and have a big damage limit.

### Frequency doubling crystals: lithium niobate (LN), sodium barium niobate, potassium niobate, α-type lithium iodate, and also various other crystals

The additional nonlinear electrical polarization coefficient is big, and also the refractive index of crystals such as LN as well as BNN is sensitive to temperature, which is various from the temperature change qualities of the dispersion result. People can readjust the temperature properly to achieve non-critical matching. Ideal for the visible light area and mid-infrared area (0.4 μ-5μ).

LN is prone to refractive index adjustment as well as photodamage under light; the damage limit of BNN is higher than that of LN, yet the strong option area is broader, and the make-up is easy to transform, leading to bad optical harmony, and also big crystals with superb efficiency are challenging to acquire; potassium niobate has no strong solution In the melting zone, it is feasible to get huge crystals with uniform optical properties; α lithium iodate is an aqueous solution development crystal, which can expand large crystals with great optical quality, and the damage limit is more than that of BNN crystals. The drawback is that it has no non-critical matching ability.

### Frequency doubling crystals: semiconductor crystals

Semiconductor crystals include gallium arsenide, gallium arsenide, zinc sulfide, cadmium zinc oxide, selenium, etc. Their square nonlinear electric polarization coefficients are higher than those of the first two crystals as well as appropriate for wider infrared bands.

Nonetheless, except for selenium and tellurium, the majority of crystals have no double refraction impact and also can not achieve placement matching.

### Frequency doubling crystals: borate, barium metaborate (β-BaB2O4), lithium triborate (LiB3O5), etc.

Amongst them, Researchers successfully established barium metaborate and also lithium triborate crystals for the first time in the 1980s. And also had the impressive benefits of huge nonlinear optical coefficients and also high laser damage threshold. It is an outstanding crystal product for laser frequency conversion, which has triggered terrific effects worldwide. Suitable for ultraviolet wavelengths, including KBF, etc, even for deep ultraviolet wavelengths. The standard demands for the amount frequency, difference frequency, and also optical specification oscillation results of nonlinear optical crystals are the same as those of dual-frequency crystals.

## BBO crystal

BBO optical crystal is a crystal with obvious comprehensive advantages and good performance in nonlinear optical crystals. It has an extremely wide light transmission range, extremely low absorption coefficient, and has a higher extinction ratio than other electro-optic modulation crystals. , large phase matching angle, high anti-light damage threshold, wide-band temperature matching and excellent optical uniformity are conducive to improving the stability of laser output power, especially for triple frequency of Nd:YAG lasers. Applications.

### The main advantages of BBO frequency doubling crystals:

• A wide range of phase-matching bands can be achieved (409.6-3500nm)
• Wide range of permeable wavelengths (190-3500nm)
• High frequency doubling conversion efficiency (equivalent to 6 times of KDP crystal)
• Good optical uniformity (δn≈10-6/cm)
• High damage threshold (1064nm, 10GW/cm2 with 100ps pulse width)
• The temperature acceptance angle is wide (around 55°C)

### The main applications of BBO crystals:

• Two, three, four and five frequency doubling of Nd:YAG and Nd:YLF lasers
• Frequency doubling, tripling and mixing of dye lasers
• Two, three and four frequency doubling of Ti:Sapphire and Alexandrite lasers
• Optical parametric amplifier (OPA) and optical parametric oscillator (OPO)
• Frequency doubling of argon ion, ruby ​​and Cu vapor lasers
• In the field of research and development in the field of high, precise and sharp laser technology such as all-solid-state tunable laser, ultrafast pulse laser, and deep ultraviolet laser