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Intracavity Frequency Doubling

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

More general term: nonlinear frequency conversion

German: Intracavity-Frequenzverdopplung

Categories: optical resonatorsoptical resonators, nonlinear opticsnonlinear optics, laser devices and laser physicslaser devices and laser physics


Cite the article using its DOI: https://doi.org/10.61835/x04

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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.

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:


[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!)

Dr. R. Paschotta

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!

Questions and Comments from Users


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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