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  or sum frequency generation. As an example, Figure 1 shows a monolithic frequency doubler, as described more in detail in Ref. . It consists of a nonlinear crystal with dielectric coatings on the end faces 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.
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.
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 e.g. appropriate optics for focusing and alignment. 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, the free spectral range of the cavity must be an integer multiple of the laser's pulse repetition rate, 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 .
Recently, enhancement cavities have been used with very intense ultrashort pulses in order 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.
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