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Intracavity Frequency Doubling

Author: the photonics expert (RP)

Definition: frequency doubling with a nonlinear crystal within the laser resonator

More general term: nonlinear frequency conversion

Categories: article belongs to category optical resonators optical resonators, article belongs to category nonlinear optics nonlinear optics, article belongs to category laser devices and laser physics laser devices and laser physics

DOI: 10.61835/x04   Cite the article: BibTex plain textHTML   Link to this page   LinkedIn

Frequency doubling, similar to other processes of nonlinear frequency conversion, can have a high power conversion efficiency only if sufficiently high optical intensities are reached in the nonlinear crystal material. This is often not possible for low- or moderate-power continuous-wave lasers. A good solution in such cases – particularly for solid-state lasers – can be intracavity frequency doubling (or intracavity SHG = second-harmonic generation), where the frequency doubler crystal is placed within the laser resonator (or similarly within the resonator of an optical parametric oscillator). There are actually two different reasons why intracavity frequency doubling works well:

  • Within the laser resonator, the optical powers and thus the intensities achievable are much higher, increasing the conversion efficiency per path by often more than an order of magnitude.
  • It is normally sufficient to achieve a single-pass conversion efficiency of just a few percent because the unconverted power remains in the laser resonator rather than being lost.
VERDI green laser
Figure 1: Photograph of the VERDI green laser from Coherent. A unidirectional ring laser resonator is used, enabling low-noise single-frequency operation. The image was kindly provided by Coherent.

It is therefore possible to generate a frequency-converted output power which is not much lower than that achievable with the same laser head at the unconverted wavelength (without a frequency doubler, and with a suitable output coupler for the fundamental wavelength).

The laser resonator must contain a dichroic mirror which has a high transmissivity for the frequency-doubled beam, and all resonator mirrors should be highly reflective for the fundamental wavelength. (In general, an ordinary output coupler mirror will not be used.) For linear resonators, frequency doubling occurs in both propagation directions. However, the two resulting beams can be combined to a single output beam when there is a folding mirror with significant transmittance for the frequency-doubled wavelength. For ideal relative phase changes on the end mirror and without parasitic losses, the double-pass conversion efficiency can be four times the single-pass value.

Concerning polarization and phase matching, there are different options:

  • A type I phase-matched frequency doubler may be used within a laser with polarized emission.
  • A type II phase-matched frequency doubler is suitable for a laser with unpolarized emission.

In both cases, the frequency-doubled light is linearly polarized.

Many continuous-wave green and blue laser sources are actually based on intracavity frequency doubling. There are versions generating tens of watts of output power. The technique is also applicable to red laser sources, based e.g. on 1342-nm vanadate lasers.

RP Resonator

Mode Calculations

The software RP Resonator is a particularly flexible tool for calculating all kinds of resonator mode properties. With a little script code (which we are happy to provide), you can also calculate power enhancement factors and the like.

Typical Technical Issues

Power Instabilities

There are some possible negative side effects of intracavity doubling in continuous-wave lasers. A very disturbing issue can be that under certain circumstances the laser can exhibit very strong intensity noise (“green problem”). This is a result of the introduced nonlinear dynamics of the resonator modes, which can be influenced not only by the nonlinear frequency conversion but also be spatial hole burning and oscillation of higher-order resonator modes. Aspects of polarization can also be important, particularly for frequency doubling with a type II phase-matched crystal.

Depending on the situation, the instabilities can be eliminated by applying different techniques in the laser design. Examples are the use of a relatively long resonator (increasing the number of oscillating resonator modes) or to design for stable single-frequency operation. (Somewhat surprisingly, single-frequency operation can even be stabilized by an intracavity doubler.)

Laser Wavelength Escaping the Nonlinear Conversion

If the gain bandwidth is larger than the phase-matching bandwidth of the nonlinear crystal, the laser wavelength may “escape” the wavelength region where the nonlinear conversion occurs, and this can result in an extremely low conversion efficiency. This problem can be eliminated with an intracavity optical filter, which essentially fixes the laser wavelength.

Excessive Fundamental Laser Power

In the case that the frequency doubler does not work, e.g. due to a crystal temperature which does not allow for phase matching, the intracavity power of the fundamental wave can become fairly high (particularly in a Q-switched laser). The design should be such that laser-induced damage of optical components is nevertheless avoided.

Effects on Pulses in Q-switched and Mode-locked Lasers

Only in rare cases is intracavity frequency doubling used with Q-switched or mode-locked lasers. This is partly because the peak powers of such pulsed lasers are anyway sufficiently high for efficient conversion, and partly because the nonlinear element can have a detrimental impact on the pulse formation. In Q-switched lasers, an intracavity frequency doubler can significantly slow the pulse build-up, and in a passively mode-locked laser it can prevent the generation of ultrashort pulses by counteracting the effect of the saturable absorber.

Other Approaches

An alternative technique, applicable to both single-frequency lasers and mode-locked lasers, is the use of a resonant enhancement cavity (→ resonant frequency doubling) external to the laser. Here, not only mode matching is required, but also precise matching of the resonance frequency.

Normally, only one of the two techniques (intracavity doubling or external resonant doubling) is used. However, Ref. [15] demonstrates the unusual combination of both techniques. Here, the frequency-doubling resonator has been placed within the laser resonator of a fiber laser. Normally, neither technique would be ideal for a high-power fiber laser: the resonator losses are too high for efficient intracavity doubling, and frequency-stabilized narrowband operation for external resonant doubling is not very convenient. The combination of both methods, however, works well: the long fiber laser resonator automatically operates on frequencies which are resonant in the short doubling resonator (a ring resonator), and the resonant enhancement takes place only in the nonlinear crystal, but not in the fiber, so that the higher losses in the fiber part are not relevant.

It is also possible to perform other kinds of nonlinear frequency conversion within a laser resonator; examples are stimulated Raman scattering, sum and difference frequency generation, and optical parametric oscillation.

More to Learn

Encyclopedia articles:

Bibliography

[1]O. Svelto and R. Polloni, “Optimum coupling for intracavity second harmonic generation”, IEEE J. Quantum Electron. 4 (9), 528 (1968); https://doi.org/10.1109/JQE.1968.1075384
[2]R. G. Smith, “Theory of intracavity optical second-harmonic generation”, IEEE J. Quantum Electron. QE-6, 215 (1970); https://doi.org/10.1109/LEOS.2006.279220
[3]T. Baer, “Large-amplitude fluctuations due to longitudinal mode coupling in diode-pumped intra-cavity doubled Nd:YAG lasers”, J. Opt. Soc. Am. B 3 (9), 1175 (1986); https://doi.org/10.1364/JOSAB.3.001175
[4]M. Oka and S. Kubota, “Stable intracavity doubling of orthogonal linearly polarized modes in diode-pumped Nd:YAG lasers”, Opt. Lett. 13 (10), 805 (1988); https://doi.org/10.1364/OL.13.000805 (polarization issue for type II doubling resolved with additional quarter-wave plate)
[5]G. E. James et al., “Intermittency and chaos in intracavity doubled lasers. II”, Phys. Rev. A 41 (5), 2778 (1990); https://doi.org/10.1103/PhysRevA.41.2778
[6]V. Magni et al., “Intracavity frequency doubling of a cw high-power TEM00 Nd:YLF laser”, Opt. Lett. 18 (24), 2111 (1993); https://doi.org/10.1364/OL.18.002111 (suppression of noise by operation on hundreds of cavity modes)
[7]M. Tsunekane et al., “Elimination of chaos in a multilongitudinal-mode, diode-pumped, 6-W continuous-wave, intracavity-doubled Nd:YAG laser”, Opt. Lett. 22 (13), 1000 (1997); https://doi.org/10.1364/OL.22.001000
[8]K. I. Martin et al., “Stable, high-power, single-frequency generation at 532 nm from a diode-bar-pumped Nd:YAG ring laser with an intracavity LBO frequency doubler”, Appl. Opt. 36 (18), 4149 (1997); https://doi.org/10.1364/AO.36.004149
[9]C. Czeranowsky, V. Baev, and G. Huber, “Stabilization of intracavity frequency-doubled lasers with type I phase matching”, Opt. Lett. 28 (21), 2100 (2003) https://doi.org/10.1364/OL.28.002100 (noise suppression by placing crystals at certain positions in the resonator)
[10]C. Du et al., “6-W diode-end-pumped Nd:GdVO4/LBO quasi-continuous-wave red laser at 671 nm”, Opt. Express 13 (6), 2013 (2005); https://doi.org/10.1364/OPEX.13.002013
[11]Q. H. Xue et al., “High-power efficient diode-pumped Nd:YVO4/LiB3O5 457 nm blue laser with 4.6 W of output power”, Opt. Lett. 31 (8), 1070 (2006); https://doi.org/10.1364/OL.31.001070
[12]L. McDonagh and R. Wallenstein, “Low-noise 62 W CW intracavity-doubled TEM00 Nd:YVO4 green laser pumped at 888 nm”, Opt. Lett. 32 (7), 802 (2007); https://doi.org/10.1364/OL.32.000802
[13]C. Stolzenburg et al., “Cavity-dumped intracavity-frequency-doubled Yb:YAG thin-disk laser with 100 W average power”, Opt. Lett. 32 (9), 1123 (2007); https://doi.org/10.1364/OL.32.001123
[14]T. Südmeyer et al., “Efficient 2nd and 4th harmonic generation of a single-frequency, continuous-wave fiber amplifier”, Opt. Express 16 (3), 1546 (2008); https://doi.org/10.1364/OE.16.001546
[15]R. Cieslak and W. A. Clarkson, “Internal resonantly enhanced frequency doubling of continuous-wave fiber lasers”, Opt. Lett. 36 (10), 1896 (2011); https://doi.org/10.1364/OL.36.001896

(Suggest additional literature!)


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This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics AG. How about a tailored training course from this distinguished expert at your location? Contact RP Photonics to find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, training) and software could become very valuable for your business!


Suppliers

The RP Photonics Buyer's Guide contains 39 suppliers for frequency doubling devices. Among them:

TOPTICA Photonics

frequency doubling devices

TOPTICA provides frequency-doubled continuous-wave diode and fiber lasers. The wavelength range covers 330 nm – 780 nm with up to 20 nm of tuning. In addition, stand-alone resonant frequency-doubling units are available.

APE

frequency doubling devices

Nonlinear frequency conversion in SHG crystals makes it possible to generate new shorter wavelengths from existing laser wavelengths. As part of our HarmoniXX product line, the HarmoniXX SHG is a frequency converter for the frequency doubling of ultrashort-pulse lasers.

The focus is on user-friendliness and a compact design. By featuring a quick exchange of optics, the HarmoniXX Second Harmonic Generation device can be used for a wide pulse duration range, from femtoseconds (fs) to several picoseconds (ps).

See our specifications for more details.

Covesion

frequency doubling devices

Covesion manufactures stock and custom free space and fiber coupled tailored to your SHG requirements. Our up-conversion solutions offer exceptional conversion efficiency for Second Harmonic Generation (SHG) of IR to visible and shorter near-IR wavelengths.

Our custom capabilities include:

  • free space or fiber coupled solutions
  • multiple grating, chirped or fan-out designs
  • tailored AR coatings
  • custom grating periods and apertures
  • 1×1 or 1×0 fiber input/output configurations
  • resistive or Peltier temperature control
  • integrated or external temperature control
  • broad wavelength coverage
  • power monitoring, control and output filtering
  • compatibility with both CW and pulsed lasers

LEDlas

frequency doubling devices

Enhance your laser systems with SHG and FHG technology, converting 1064 nm to 532 nm and further to 266 nm. Achieve high-efficiency green and UV outputs for precision applications in research, industry, and medical fields.

HC Photonics

frequency doubling devices

HC Photonics (HCP) offers both commercial off-the-shelf (COTS) and custom PPMgO:LN or PPMgO:LT based frequency doubling devices for wavelength ranging from 710 nm to 5000 nm.

The product mix includes bare crystals (bulk and waveguide) and plug-and-play fibered mixers. Key features:

  • >200 kinds of commercial off-the-shelf (COTS) crystals with oven/holder for shipping today
  • wavelength range: 710–5000 nm (355–2500 nm output after doubling)
  • fibered mixer is single-pass, high-efficiency and optimized for specified input pumps
  • available for fiber or free space as input/output coupling interfaces (such as 1×0, 1×1 or 0×0; 0 = free space, 1 = one fiber)
  • >2W CW 780 nm 1×1 fibered waveguide mixer is an excellent device for quantum application
  • >4 W visible 1x0 fibered bulk mixer is a robust, compact and easy-to-use device for biophotonics, metrology and quantum applications

Radiantis

frequency doubling devices

Radiantis offers second harmonic generators for MHz repetition-rate femtosecond and picosecond Ti:sapphire oscillators with conversion of efficiencies of 50%. The company also provides femtosecond SHG devices to double the 990 – 1550 nm range into the 495 – 775 nm range. Products are fully automated. Customised frequency doublers for other spectral ranges can also be developed.

EKSMA OPTICS

frequency doubling devices

Our nonlinear crystals for second harmonic generation include a wide choice of BBO, LBO, DKDP, KDP and KTP crystals for quick delivery. We also offer AgGaS2, AgGaSe2, GaSe, ZnGeP2, LiIO3 and KTA crystals for SHG at IR and mid-IR wavelengths. We also offer technical consulting services helping to choose and specify optimal crystal material, orientation and coating design for your particular application.

GWU-Lasertechnik

frequency doubling devices

GWU's frequency conversion devices are flexible solutions for pulsed lasers. By means of second harmonic generation (SHG), third harmonic generation (THG) and fourth harmonic generation (FHG), not only the wavelength range but also the versatility of laser sources can be vastly extended. The UHG series offers widely tunable harmonic generation in a compact, modular design for ultrafast laser oscillators. Broadband optics for complete wavelength coverage ensure convenient usability and best performance.

NKT Photonics

frequency doubling devices

The Koheras HARMONIK high-power frequency-converted laser systems are single-frequency, mode-hop-free, and ultra-stable. Engineered for high-power and narrow-linewidth light at precise atomic transitions, they are perfect for quantum applications. Experience excellence with Koheras HARMONIK HP laser systems, ideal for quantum computing, sensing, metrology, and communications. Featuring a robust single-frequency design, low phase and intensity noise, high OSNR, and industrial-grade reliability.

Shalom EO

frequency doubling devices

Hangzhou Shalom EO offers a vast selection of nonlinear optical crystals for frequency doubling, also known as second harmonic generation (SHG). 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.

Questions and Comments from Users

2023-12-30

Is there an ideal frequency operating range for frequency doubling crystals?

Is it possible to multiply the frequency output even more (e.g. 4×, 8× or 16×) by using a series of consecutive frequency doubling crystals? Would the consecutive crystals have to be of different composition?

The author's answer:

In principle, you could have multiple frequency doublers in a resonator to reach higher optical frequencies, but this is rather difficult to realize in practice:

  • For shorter wavelengths, it gets more difficult to find suitable nonlinear crystal materials which have
    • good enough transmission,
    • long enough endurance, and
    • can be phase-matched with reasonable properties (e.g. without excessive spatial walk-off).
  • (Even just for reasons of phase matching, it is likely that you would need different crystal materials.)
  • It is difficult to have simultaneous resonant enhancement at multiple frequencies; that would require complicated stabilization schemes. If only the fundamental wave is resonant, it gets hard to obtain sufficiently high conversion efficiency for the second and any further frequency doubling steps.

Note, however, that there are methods of high harmonic generation where you get a substantial number of harmonics, albeit with relatively low powers and with rather high requirements on the laser source.

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