Lifetime of Nonlinear Crystals

Introduction

In most cases, nonlinear crystals used for nonlinear frequency conversion have very long lifetimes, longer than the lifetime of bulk lasers. Crystals are essentially unaltered in use. However, crystal lifetime reduction can occur in various situations: eg in nonlinear frequency conversion.
Too high a light intensity during operation will damage the crystal in seconds. Unfortunately, to achieve sufficiently high conversion efficiencies, nonlinear crystals often have to be operated near their optical loss threshold. This implies a trade-off between conversion efficiency and crystal lifetime. It is important to note that problems may still exist even if the nominal intensity is less than the symbolic damage threshold.
Even operating the crystal well below threshold in order to avoid transient wear and tear, some crystal materials exhibit continuous degradation in some used parts, for example, in the form of ‘gray tracking’. This situation is especially prevalent when the crystal is operating in the UV range. Of concern is that gradual degradation builds up heat, and the generation of overheating can cause catastrophic damage in an instant.
Hygroscopic crystalline materials deteriorate when they cannot be maintained in sufficiently dry air (or dry inert gas). This crystal is used in KDP and BBO, less used in LBO. It is helpful to keep the crystals at a relatively high temperature, which makes it easier to keep the crystals dry.
Operating nonlinear crystals below room temperature in order to achieve phase matching is often problematic because that can cause water to condense on the crystal surface if the surrounding air is not very dry. Even if the crystal material or coating is not water-sensitive, tiny water droplets can damage the crystal material by focusing the laser more strongly than usual.
Noncritical phase-matched crystals in crystal ovens can have problems when the temperature changes rapidly or frequently. In particular, antireflective coatings can be damaged due to different material expansion coefficients.
Although the degradation phenomenon seems to be an intrinsic limitation of the material, the crystal lifetime is largely dependent on the material quality.
For high-power UV generation, nonlinear crystals become consumables: they need to be replaced frequently during the lifetime of the laser system (eg, every few hundred hours of operation). Often, several problematic factors come together in UV-generating systems: crystalline materials are generally more sensitive to UV light (with high photon energies), and in this system exhibit higher absorptivity, in the case of ultrashort pulses, A high group velocity mismatch requires the use of shorter crystals, which require higher light intensities for the same conversion efficiency.