High Power Lasers on the Basis of Yb- and Nd-doped Materials (1020 – 1080 nm)

Yb- and Nd-doped high power lasers are commonly used in material processing as well as measurement applications. Thin disk, slab and fiber lasers were developed to achieve high cw-output power and excellent beam quality. They can also be operated as high power lasers in the ns to fs range.

Mirrors

Fig.1:Reflectance spectra of HR mirrors for 1030 nm

a)Cavity mirror

b)Turning mirror

LAYERTEC has developed different coating designs for handling extraordinarily high power densities or energy densities in cw lasers or pulsed lasers, respectively. The designs are optimized either for cw radiation, ns pulses or ps pulses.

Edge Filters and Pump Mirrors

Fig.2:Reflectance spectrum of a steep edge short pass filter which can be used as pump mirror for fiber lasers

Fig.3:Reflectance spectrum of a steep edge long pass filter
HR (0°, 915 – 980 nm) > 99.8 % + R (0°,1030 – 1200 nm) < 3 %
for use as output mirror of a fiber laser (rear side AR coated)

Special Features

Short pass filters with a very steep edge which are utilized as a pump mirror for solid-state lasers based on Yb-doped materials, e.g. Yb:YAG, Yb:KGW or Yb-doped fiber
Also useful for Nd-doped and Yb-Nd-co-doped materials

Thin Film Polarizers

Thin film polarizers are key elements for regenerative amplifiers in ns- and ps-lasers. LAYERTEC optimizes its polarizer designs for high laser-induced damage thresholds (LIDT). Fig. 4 shows examples of a broadband polarizer with Rp < 0.2 % for AOI = 55° with a bandwidth of 25 nm in the wavelength range of Yb-doped fiber lasers (Fig. 4a) and a narrowband polarizer which is optimized for very low Rp values at a single wavelength (Fig. 4b). Fig. 4c shows a comparison of the calculated transmittance spectrum for p-polarized light and a measurement in a CRD set-up. Fig. 4d shows Tp vs. AOI for the same polarizer (measured at 1042 nm). These measurements prove that Tp > 99.9 % can be achieved by angle adjustment. This is important especially for intra cavity applications.

Fig.4:Reflectance and transmittance spectra of thin film polarizers (TFPs)

a)Reflectance spectra for s- and p-polarized light of a broadband thin film polarizer showing a bandwidth of 25 nm with Rp < 0.2 % (AOI = 55°)

b)Reflectance spectra for s- and p-polarized light of a narrowband thin film polarizer which is optimized for very low Rp values and easy angle adjustment for the optimization of the polarizer performance (AOI = 55°)

c)Calculated and measured transmittance spectra for p-polarized light of the narrowband thin film polarizer shown in Fig. 4b (AOI = 55°). Tp > 99.8 % is reached with a bandwidth of 15 nm and Tp > 99.9 % can be achieved within a bandwidth of 5 nm. The spectral position of this transmittance maximum can be adjusted to any wavelength between 1035 nm and 1045 nm by angle adjustment.

d)Transmittance spectrum Tp vs. AOI at 1042 nm measured at the polarizer shown in Fig. 4c

Picosecond Lasers based on Yb-doped Materials

Picosecond laser optics require specially designed coatings to achieve high laser damage thresholds. For detailed information please see page High Power Ultrafast Laser Optics (550 – 1100 nm) ff. For GTI mirrors which are often used for pulse compression from the ps range down to a few hundred fs please see page Gires-Tournois-Interferometer (GTI) Mirrors (600 – 1600 nm).

Alignment and Process Monitoring

Fig.5:Reflectance spectra of turning mirrors with special features for alignment and process monitoring

a)Turning mirror with low IR reflectance, i.e. high IR transmittance for process monitoring

b)Silver based turning mirror with Ru (45°, 1064 nm) > 99.8 % and with Ru > 80 % for an alignment laser in the red spectral range

Output Couplers and Beam Splitters

Fig.6:Reflectance spectra of output couplers and beam splitters (center wavelength 1064 nm)

a)Output couplers with different degrees of reflectance

b)Common 50:50 beam splitter for unpolarized light

Beam splitters and output couplers can be produced with a precisely adjusted degree of reflectance according to customer specifications.
Common types:
- Rs,p > 95 % ±0.5 %
- Rs,p = 80 % … 95 % ±1 %
- Rs,p = 10 % … 80 % ±2 %

Non Polarizing Beam Splitters

Fig.7:Non-polarizing beam splitters

a)Calculated reflectance spectra of 3 types of non-polarizing beam splitters for AOI = 45°

b)Measured reflectance spectra of the 50 % beam splitter

Beam splitters with Rs ≈ Rp (|Rs – Rp| < 1.5 %) for AOI = 45° and different degrees of reflectance:
Common types:
- Rs,p = 66 % ±1 %
- Rs,p = 50 % ±2 %
- Rs,p = 33 % ±3 %
All non-polarizing beam splitters with rear side AR:
Rs ≈ Rp ≤ 0.6 %

High Power Lasers on the Basis of Yb- and Nd-doped Materials (1020 – 1080 nm)