This article introduces what frequency tripling are, frequency tripling devices, and common frequency tripling applications.

An Effect Using Nonlinear Materials to Achieve Laser Frequency Three Times the Input Light Frequency

Frequency doubling is a nonlinear frequency transformation that results in light having a frequency that is three times the frequency of the input laser. In principle, this enables direct third-harmonic generation via χ(3) nonlinearity, but the small χ(3) of the medium and the limitations of phase matching make it difficult to achieve frequency triples with this method.

Therefore, the frequency triple is usually generated by a cascaded nonlinear process. First, the input beam is frequency doubled, and then the frequency triple process is realized by using the sum frequency of the frequency doubled light and the fundamental frequency light. Both processes are based on nonlinear crystals. The χ(2) of the material is nonlinear.

A typical frequency tripler setup: An infrared input with a wavelength of 1064nm uses a frequency doubler to achieve green rice at a wavelength of 532nm, and then uses a sum frequency to achieve 355nm light.

The main application of frequency tripling is the generation of ultraviolet light. The most common is to use the 1064nm laser generated by Nd:YAG or Nd:YVO4 laser to achieve frequency triple, resulting in ultraviolet light with a wavelength of 355nm. A common approach is to use two LBO (lithium triborate) crystals, or one LBO and one BBO crystal, one for phase-matched second harmonic generation and the other for sum-frequency process.

Using a Q-switched or mode-locked laser can make this process very efficient, of course, using intracavity frequency doubling and sum frequency process can also achieve efficient frequency triple process of continuous light.

In addition, the generation of blue light can also be achieved by using the frequency triple of the laser output of the neodymium-doped laser with a wavelength of 1.3um.

Theoretically, the power conversion efficiency of the frequency triple process can be close to 100%. When the frequency doubler has a conversion efficiency of 2/3, then the power of the frequency doubled light is twice that of the remaining pump light, so that the two beams of light have the same number of photons. But in practice, the efficiency of the frequency doubler is usually slightly lower (usually about 40 to 50%), and the efficiency of the sum frequency mixer is much lower than 100%.

There are many reasons for the latter, such as lower light intensity, design constraints due to optical damage, spatial walk-off, mismatch of pulse widths, temporal walk-off, etc. The best conversion effect is when the input optical peak power is high and the pulse width is wide, the beam quality is high, and the bandwidth is narrow. In this case, the total conversion efficiency from infrared light to ultraviolet light can reach the order of 30-40%.

In order to achieve efficient third harmonic generation, nonlinear crystals usually need to withstand relatively high light intensities. But for sum-frequency processes, this often creates a problem: even for beams with light intensity below the damage threshold, intense UV light can cause gradual degradation of the nonlinear crystal and the AR coating on the exit surface. In this case, the nonlinear crystal often becomes a consumable item, which means that the operating life of the system is rather limited, requiring crystal replacement. Since this effect is usually only affected by the beam diameter, the effect of this effect can be reduced by moving the nonlinear crystal.

The final crystal lifetime depends on various factors, including the type of nonlinear crystal material, material quality, magnitude of peak and average light intensities, type of AR coating, pulse parameters (such as pulse width), and the degree of contamination of the ambient air . For example, oils from mechanical parts undergo chemical changes under the influence of UV light, which can lead to their deposition on crystalline materials.

It should be noted that if the mode field area on the crystal is proportionally increased, the enhancement of the UV light power will not lead to a decrease in the working life of the crystal.

For a given output power level, a substantial increase in crystal lifetime can be achieved by preventing third harmonic crystals in the boost cavity. The single-pass conversion efficiency is reduced by frequency conversion using resonance, while the pump light is “recovered” as non-converted. . However, resonance-enhanced frequency doubling is not always possible (eg, due to insufficient coherence of the laser source), and also introduces additional complexity to the system.