enhancement cavities (original) (raw)
Definition: optical cavities for exploiting the resonant enhancement of the power of circulating light
Alternative term: resonant enhancement cavities
Categories: 
optical resonators, 
photonic devices
Related: cavitiesnonlinear frequency conversionresonant frequency doublingintracavity frequency doubling
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DOI: 10.61835/l3t Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn
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Contents
What are Enhancement Cavities?
An enhancement cavity is an optical cavity (resonator) which is used for resonant enhancement of an optical power or intensity: if the incident light is resonant with the cavity and is mode-matched to it, the intracavity power can be far above the incident power, particularly for a cavity with high finesse, and that aspect is typically exploited for some purpose — typically, for efficient nonlinear frequency conversion. The topography of the cavity can be that of a linear cavity or a ring resonator.
An enhancement resonator can contain other optical components. For example, a nonlinear crystal can be used for efficient nonlinear frequency conversion, such as frequency doubling [1] or sum frequency generation. As an example, Figure 1 shows a monolithic frequency doubler, as described in more detail in Ref. [5]. It consists of a nonlinear crystal with dielectric coatings on the endfaces which make the pump wave (red) resonant. Frequency-doubled light is extracted on the right-hand side. Even if the nonlinear process converts only a small fraction of the circulating optical power, the resonator allows for a kind of recycling of the unused light. The conversion can be very efficient if impedance matching is achieved, i.e. if the input mirror transmission equals all other resonator losses.
Figure 1: Monolithic resonant frequency doubler with dielectric coatings on the curved endfaces.
Frequency doubling can be efficient at even significantly lower powers (a few milliwatts) by using a doubly resonant scheme, where both pump wave and second-harmonic wave are resonant. However, the double resonance is usually delicate to maintain.
Resonant doubling should not be confused with intracavity frequency doubling, where the nonlinear crystal is placed within the laser cavity, so that there is no need for a separate resonant cavity.
Figure 2: A ring resonator, used as enhancement cavity for nonlinear frequency conversion.
Resonator Optimization
The software RP Resonator is a particularly flexible tool for optimizing optical resonators, including enhancement cavities. For example, you can check the astigmatism induced by non-perpendicular beam incidence on the mirrors and effects of using a crystal at Brewster's angle. With a little script code, you can also calculate the power enhancement factor.
Conditions for Efficient Resonant Enhancement
For efficient operation of an enhancement cavity, several factors have to be considered:
- The resonance condition can usually be fulfilled only for single-frequency light, or for a frequency comb (see below). The resonance condition leads to the requirement that the resonator length must be correct within a fraction of an optical wavelength. An electronic feedback loop is often used for maintaining the resonance over longer times. Such a feedback loop may either adjust the optical frequency of the laser to match the cavity frequency, or adjust the cavity length e.g. via a piezo actuator below a resonator mirror. Note that for high-finesse cavities, which make possible a large power enhancement, the stability requirements on both the cavity and the laser can be very high.
- The incoming radiation must be spatially mode-matched to the cavity, using appropriate optics for focusing and alignment, for example. The incoming light must usually be delivered with essentially diffraction-limited beam quality.
- To minimize losses via back reflection of pump power, the enhancement cavity should be impedance matched. This means that the transmission coefficient of the input mirror for the pump radiation matches the coefficient quantifying all other losses.
Enhancement Cavities for Mode-locked Lasers
Enhancement cavities are often applied in conjunction with single-frequency lasers, but can also be used with mode-locked lasers. In the latter case, the cavity length has to be chosen such that the cavity round-trip time is an integer multiple of the pulse spacing. In other words, laser's pulse repetition rate must be an integer multiple of the free spectral range of the cavity, so that all lines of the laser output (→ frequency combs) can be simultaneously resonant. Also, the intracavity chromatic dispersion and nonlinearity should not be too strong [8].
Recently, enhancement cavities have been used with very intense ultrashort pulses to obtain high harmonic generation at very high pulse repetition rates [6, 7]. Challenges arise from the need for precise intracavity dispersion compensation, from the very high optical intensities on resonator mirrors and other optics, and from beam distortions due to plasma generation in the gas used for the high harmonic generation.
Frequently Asked Questions
What is an enhancement cavity?
An enhancement cavity is an optical resonator used to resonantly increase optical power or intensity. If an incident laser beam is resonant with and mode-matched to the cavity, the power circulating inside can be far greater than the incident power.
What are the main applications of enhancement cavities?
They are typically used to reach very high optical intensities for processes like efficient nonlinear frequency conversion, for example in frequency doublers, or for high harmonic generation.
What key conditions are required for efficient power enhancement?
Three conditions are essential: the laser frequency must be resonant with the cavity, the incident beam must be spatially mode-matched to the cavity mode, and the cavity must be impedance-matched.
What does impedance matching mean for an enhancement cavity?
Impedance matching is the condition where the transmission of the input mirror equals the sum of all other round-trip losses in the cavity. This minimizes reflection from the cavity and maximizes the intracavity power.
Can enhancement cavities be used with pulsed lasers?
Yes, when used with a mode-locked laser, the laser's pulse repetition rate must be an integer multiple of the cavity's free spectral range. This ensures that all the frequency components of the laser's frequency comb are simultaneously resonant.
Suppliers
Sponsored content: The RP Photonics Buyer's Guide contains two suppliers for enhancement cavities. Among them:
âš™ hardware
UltraFast Innovations (UFI®) offers passive enhancement cavities to achieve effective nonlinear conversion even in extreme spectral regions. Typical applications include XUV generation via higher harmonic generation in a gas at megahertz pulse repetition rates and hard X-ray generation via Thomson (inverse Compton) scattering of relativistic electrons. The cavities also provide longitudinal and transverse mode filtering and can be used for the highly precise characterization of laser optics concerning amplitude and phase.
Bibliography
| [1] | A. Ashkin, G. D. Boyd and J. M. Dziedzic, “Resonant optical second harmonic generation and mixing”, IEEE J. Quantum Electron. 2 (6), 109 (1966); doi:10.1109/JQE.1966.1074007 |
|---|---|
| [2] | B. Couillaud, T. W. Hänsch and S. G. MacLean, “High power CW sum-frequency generation near 243 nm using two intersecting enhancement cavities”, Opt. Commun. 50, 127 (1984); doi:10.1016/0030-4018(84)90149-4 |
| [3] | Z. Y. Ou and H. J. Kimble, “Enhanced conversion efficiency for harmonic-generation with double-resonance”, Opt. Lett. 18 (13), 1053 (1993); doi:10.1364/OL.18.001053 |
| [4] | K. Fiedler et al., “Highly efficient frequency-doubling with a doubly resonant monolithic total-internal-reflection ring resonator”, Opt. Lett. 18 (21), 1786 (1993); doi:10.1364/OL.18.001786 |
| [5] | R. Paschotta et al., “82% efficient continuous-wave frequency doubling of 1.06 μm with a monolithic MgO:LiNbO3 resonator”, Opt. Lett. 19 (17), 1325 (1994); doi:10.1364/OL.19.001325 |
| [6] | R. J. Jones and J. Ye, “High-repetition-rate coherent femtosecond pulse amplification with an external passive optical cavity”, Opt. Lett. 29 (23), 2812 (2004); doi:10.1364/OL.29.002812 |
| [7] | R. J. Jones et al., “Phase-coherent frequency combs in the vacuum ultraviolet via high harmonic generation inside a femtosecond enhancement cavity”, Phys. Rev. Lett. 94 (19), 193201 (2005); doi:10.1103/PhysRevLett.94.193201 |
| [8] | K. D. Moll et al., “Nonlinear dynamics inside femtosecond enhancement cavities”, Opt. Express 13 (5), 1672 (2005); doi:10.1364/OPEX.13.001672 |
| [9] | K. D. Moll et al., “Output coupling methods for cavity-based high harmonic generation”, Opt. Express 14 (18), 8189 (2006); doi:10.1364/OE.14.008189 |
| [10] | D. C. Yost et al., “Efficient output coupling of intracavity high harmonic generation”, Opt. Lett. 33 (10), 1099 (2008); doi:10.1364/OL.33.001099 |
| [11] | R. Krischek et al., “Ultraviolet enhancement cavity for ultrafast nonlinear optics and high-rate multiphoton entanglement experiments”, Nature Photon. 4 (3), 170 (2010); doi:10.1038/nphoton.2009.286 |
| [12] | I. Pupeza et al., “Power scaling of a high-repetition-rate enhancement cavity”, Opt. Lett. 35 (12), 2052 (2010); doi:10.1364/OL.35.002052 |
| [13] | H. Carstens et al., “Large-mode enhancement cavities”, Opt. Express 21 (9), 11606 (2013); doi:10.1364/OE.21.011606 |
| [14] | N. Lilienfein et al., “Enhancement cavities for few-cycle pulses”, Opt. Lett. 42 (2), 271 (2017); doi:10.1364/OL.42.000271 |
(Suggest additional literature!)
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