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Bauteile für höhere Harmonische der Nd:YAG und Nd:YVO4 Laser

The harmonics of Nd:YAG and Nd:YVO₄ lasers are widely used for materials processing as well as for measurement applications. On these pages we introduce dual wavelength mirrors and separators for the fourth (266nm) and fifth harmonic (213nm). All designs are calculated according to customer specification.

Figure 1: ReflectaReflectance spectra of a dual wavelength turning mirror for 266nm and 355nm (a) and a separator for the fourth harmonic from the long wavelength harmonics and the ground wavelength (b)

Figure 2: Measured reflectance spectra of a turning mirror for the fifth harmonic (a) and of a separator for the fifth harmonic from the long wavelength harmonics and the ground wavelength (b); fluoridic coatings on CaF₂

Separator type	Centre wavelength	Reflectance at centre wavelength [%]		Reflectance at the corresponding longer Nd:YAG wavelengths [%]
				266nm		355nm		532nm		1064nm
		R_s	R_p	R_s	R_p	R_s	R_p	R_s	R_p	R_s	R_p
3rd harmonic	355nm	> 99.7	> 99					< 5	< 2	< 10	< 2
4th harmonic	266nm	> 99.7	> 99			< 5	< 2	< 10	< 2	< 10	< 2
5th harmonic	213nm	> 97	> 93	< 5	< 2	< 10	< 2	< 10	< 2	< 10	< 2

Table 1:	Common specifications of separators for the harmonics in the UV
*	Fluoridic coating on CaF₂

Figure 3: Reflectance spectra of separators for the second harmonic from the fourth harmonic:
(a): HR (0°, 532nm) > 99.5% + R (0°, 266nm) < 10% (rear side uncoated)
(b): HR (0°, 532nm) > 99.9% + R (0°, 266nm) < 5% (rear side uncoated)

Components for 266nm are normally produced by e-beam evaporation. These coatings exhibit high damage thresholds, but the reflectivity in the visible spectral range is limited to R > 99.5% (see figure 3a). Recent research results on sputtered coatings for the UV show, that separators with a significantly improved reflectivity in the VIS can be produced, which have similar properties in the UV (see figure 3b).

Figure 4: Reflectance spectra of separators for the fourth and fifth harmonics:
HR_r (45°, 266nm) > 98% + R_r (45°, 213nm) < 10% for unpolarized light
a) oxide coatings optimized for low straylight losses (rear side uncoated)
b) fluoride coatings for high laser induced damage thresholds (rear side uncoated)

The fifth harmonic at 213nm is a critical wavelength for oxide coatings, because the absorption edge of aluminum oxide begins in this wavelength range. For high power applications we recommend fluoridic coatings on calcium fluoride which are produced according to the technology of ArF-excimer laser mirrors.

Thin Film Polarizers

Figure 5: RReflectance spectra of thin film polarizers for 266nm (a) and 213nm (b):
a) HR_s(56°, 266 nm) > 98 % + R_p(56°, 266 nm) < 1 %
b) HR_s(56°, 213 nm) > 97 % + R_p(56°, 213 nm) < 5 %;
Please note that T_p ist only about 75% because of the above mentioned absortion losses

Magnetron sputtering enables us to offer thin film polarizers also for the fourth and fifth harmonic of the Nd:YAG laser.