frequency tripling (original) (raw)

Definition: the phenomenon that an input laser beam generates a beam with three times the optical frequency

Alternative term: third-harmonic generation

Category: nonlinear optics

• nonlinear frequency conversion devices

  • frequency multiplication
    * frequency tripling
    * optical frequency doublers
    * triplers
    * quadruplers

• nonlinear optical effects

  • nonlinear frequency conversion
    * frequency doubling
    * frequency tripling
    * frequency quadrupling
    * sum and difference frequency generation
    * optical rectification
    * supercontinuum generation
    * optical parametric oscillation and generation
    * stimulated Raman scattering
    * intracavity frequency doubling
    * optical frequency multipliers
    * high harmonic generation
    * (more topics)

Related: frequency doublingfrequency quadruplingnonlinear frequency conversionnonlinear crystal materialsultraviolet lightultraviolet lasers

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DOI: 10.61835/hnl Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What is Frequency Tripling?

Frequency tripling is a process of nonlinear frequency conversion where the resulting optical frequency is three times that of the input laser beam. In principle, that can be achieved with a ($\chi^{(3)}$) nonlinearity for direct third-harmonic generation [12, 13, 14], but this is difficult due to the small ($\chi^{(3)}$) nonlinearity of optical media and also because of phase-matching constraints (except for tripling in gases). Therefore, frequency tripling is usually realized as a cascaded process, beginning with frequency doubling of the input beam and subsequent sum frequency generation of both waves, with both processes being based on nonlinear crystal materials with a ($\chi^{(2)}$) nonlinearity.

Figure 1: A typical configuration for frequency tripling: an infrared input beam at 1064 nm generates a green 532-nm wave, and these two mix in a second crystal to obtain 355-nm light.

Applications

The main application of frequency tripling is the generation of ultraviolet light. Most common is the generation of 355-nm light by frequency tripling of a laser beam with 1064 nm, as obtained from a Nd:YAG or Nd:YVO4 laser. A common approach is to use two LBO (lithium triborate) crystals, or an LBO and a BBO crystal, the first being phase-matched for second-harmonic generation and the second for sum frequency generation. It is easy to make this process efficient when using pulses from a Q-switched or mode-locked laser, but also possible in continuous-wave operation e.g. with intracavity frequency doubling and resonant sum frequency generation.

It is also possible to generate blue light by frequency tripling the output of a 1.3-μm neodymium laser.

Power Conversion Efficiency

Theoretically, the total power conversion efficiency of the frequency tripling process could be close to 100 % in a single pass through the crystals. For that, the frequency doubler should have a conversion efficiency of 2/3, so that the second-harmonic wave has twice the power of the remaining fundamental wave, and both have equal photon numbers. In practice, the efficiency of the frequency doubler is normally somewhat lower (often around 40 to 50 %), and in particular the sum frequency mixer is far from 100 % efficient. The latter problem can result from many effects, such as too low optical intensities, design limitations enforced by optical damage, effects of spatial walk-off, mismatch of pulse duration and/or temporal walk-off, etc. Tentatively, the conversion works best for high peak powers in not too short (e.g. picosecond) pulses, and when the beam quality is high and the optical bandwidth not too high. Overall conversion efficiencies from infrared to ultraviolet can then be of the order of 30 to 40%.

With numerical modeling, the whole frequency tripling process can be simulated with fairly high accuracy and reliability of the results.

Degradation of Nonlinear Crystals

For efficient single-pass third-harmonic generation, the nonlinear crystals need to be operated with fairly high optical intensities. For the sum-frequency mixer crystal, this is often a problem: the intense ultraviolet light can lead to gradual degradation of the crystal material and an anti-reflection coating on the exit surface, even for operation well below the threshold for instant laser-induced damage. Under such conditions, nonlinear crystals can become consumables, i.e., system parts with quite limited lifetime, which need to be replaced periodically. As normally only the volume of the beam is affected, one may often somewhat shift the crystal to use another region which has not yet been degraded. Such movements may also be done with some automatic crystal shifter; this method is sometimes called indexing of the crystal. Under good conditions, one may reach several hundred hours of operation on one spot of an LBO crystal and use dozens of spots for obtaining an overall lifetime of many thousand hours with a single crystal.

The achieved crystal lifetime can depend on various factors, including the crystal material type, the material quality, the peak and average intensity levels, details of anti-reflection coatings, other pulse parameters such as the pulse duration, and also contaminants in the ambient air. For example, hydrocarbons from some oils at mechanical parts may be chemically altered by the ultraviolet light and may lead to the deposition of absorbing material on the crystal and on other optics, e.g. a collimation lens.

Note that higher UV output powers do not necessarily decrease the crystal lifetime, if the mode areas in the crystal are increased in proportion to the power level. The operational intensities are then not higher than in lower-power devices. One may be tempted, however, to use higher powers for applying higher intensities with the goal of further increasing the power conversion efficiency. This can then reduce the crystal lifetime.

Substantial improvements in crystal lifetime for a given output power level are possible by using an enhancement cavity around the third-harmonic crystal, i.e., by doing resonant frequency conversion. This is because the single-pass conversion efficiency can then be lower, as the non-converted light is “recycled”. One may then work without an automatic crystal shifter. However, resonant enhancement is not always possible (e.g., for laser sources with insufficient coherence properties) and introduces additional complications.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is frequency tripling?

How is frequency tripling commonly achieved?

Frequency tripling is most often realized in two steps using nonlinear crystal materials. First, a portion of the input beam is converted to its second harmonic. Then, this second-harmonic beam and the remaining fundamental beam are mixed in another crystal to produce the third harmonic.

What is the main application of frequency tripling?

The primary application of frequency tripling is the generation of ultraviolet light. A common example is producing 355-nm UV light by tripling the 1064-nm output of Nd:YAG or Nd:YVO4 lasers.

How efficient is the frequency tripling process?

While theoretically possible to reach nearly 100% efficiency, practical single-pass systems are limited by various factors. For high-peak-power pulsed lasers, overall conversion efficiencies from infrared to ultraviolet are typically in the range of 30% to 40%.

Why is crystal lifetime an issue in frequency tripling?

The intense ultraviolet light generated can cause gradual degradation of the nonlinear crystal used for sum frequency generation. This limits the crystal's lifetime, sometimes requiring periodic replacement or shifting the beam to an unused spot on the crystal.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains 22 suppliers for frequency tripling. Among them:

💡 consulting🧰 development🎓 training

We offer advice on all aspects of nonlinear frequency conversion, e.g. the design of frequency conversion devices, choice of nonlinear materials, simulation of nonlinear conversion.

We also offer specialized in-house training courses, tailored to your needs.

⚙ hardware

We offer the Femtokits — a complete solution for efficient third harmonic generation with femtosecond Ti:sapphire lasers in laboratory settings. There is a wide choice of standard BBO, LBO and DKDP nonlinear crystals for THG in the Nd:YAG LaserLine and FemtoLine product groups.

⚙ hardware🧩 accessories and parts🧴 consumables🔧 maintenance, repair📏 metrology, calibration, testing💡 consulting🧰 development

Shalom EO offers a vast selection of nonlinear optical crystals for frequency tripling, also known as third harmonic generation (3 HG). Shalom EO supplies different kinds of NLO crystals including BBO crystals, CLBO crystals, KDP and KD*P, KTA, PPLN waveguides, LBO, KTP, HGTR KTP, BIBO, LiNbO3 and MgO:LiNbO3 crystals, YCOB, in addition to the infrared nonlinear crystals ZnGeP2 (ZGP) and LiIO3. Miscellaneous coating options including uncoated, AR, HR, HT, PR coatings, electrodes, and custom coatings are available.

Both off-the-shelf and customized products are available. The crystals can be offered in the form of ingots, blanks, and laser-grade polished elements.

Recent days, Shalom EO launched CLBO (cesium lithium borate) crystals. Processed and polished in low humidity workshop, our CLBO crystals feature excellent performance in UV, vacuum UV (VUV), and deep UV (DUV) wavelength ranges.

Ultra-thin nonlinear crystals for femto-line lasers are also available.

⚙ hardware

As part of our HarmoniXX product line, the HarmoniXX THG is a frequency converter for tripling the frequency of ultra short-pulse lasers.

Unlike conventional triplers, the HarmoniXX THG requires less adjustment efforts, because no separation and recombination of the interacting beams is needed. This user-friendly feature has one common optical beam path for all interacting beams. It is implemented by means of a proprietary delay compensator and provides consistent spatial overlap for optimum efficiency.

For more details, see our specifications.

⚙ hardware

HCP offers both commercial off-the-shelf (COTS) and custom frequency tripling devices based on PPMgO:LN or PPMgO:LT crystals, covering input wavelengths from 1064 to 5000 nm.

The product portfolio includes bare crystals as well as plug-and-play fiber-coupled mixers. A well-received third harmonic generation (THG) solution — based on SHG followed by SFG — delivers ≥50 mW at 355 nm using a 3.5 W CW pump at 1064 nm, and 300 mW at 520 nm by 1560 nm. Both are available in either fiber-coupled or free-space formats.

Bibliography

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