High Power Ultrafast Laser Optics (550 – 1100 nm)

High Power Ultrafast Laser Optics

Ultrafast lasers are widely used in measurement applications and materials science. Ultrafast lasers enable the machining of metals as well as of dielectric materials by cold, i.e. non-thermal, processes. The most important feature of these treatment steps is the avoidance of melt. That is why pieces machined with ultrafast lasers are of high accuracy and do not require mechanical postprocessing. The demands for efficient production processes drive the development of high power fs lasers. In most cases, these lasers show pulse lengths between 100 fs and 1 ps.

Moreover, high power ultrafast lasers with power levels in the terawatt and petawatt range become more and more important in basic research on light material-interaction, particle physics and even for medical applications. The pulse duration of these lasers is considerably shorter than that of lasers for material processing. Typical pulse durations range from 20 fs to 50 fs.

The laser types mentioned above require optics with high laser-induced damage thresholds (LIDT). High power coatings for ultrafast lasers were the topic of a number of scientific investigations in the last years [1, 2]. Research institutes as well as optics manufacturers have spent much effort on the improvement of the LIDT of fs laser optics. LAYERTEC has dealt with this issue for more than 20 years.

The main result of these investigations mentioned above was that the LIDT of optical coatings in the fs regime is strongly related to the band gap of the coating materials as well as the coating designs. Materials with larger band gaps exhibit larger LIDT. However, there is a trade-off between damage threshold and bandwidth, as large band gaps also translate into a smaller difference of the refractive indices. Thus, turning mirrors made of these materials only have a bandwidth of about 100 nm for p-polarized light at AOI = 45°. This bandwidth is sufficient for pulse lengths as low as 25 fs. Please note that all LAYERTEC high power designs are optimized for GDD < 50 fs².

In contrast, materials with a large difference of the refractive indices may be used in order to achieve large bandwidths. Designs for standard low-GDD components exhibit medium LIDT values, whereas broadband designs result in low damage thresholds. This is also the case when considering mirrors with dispersion control, such as chirped mirror pairs or GTI mirrors. Here, bandwidth and phase requirements outweigh LIDT. However, depending on the complexity of the overall constraints, some optimization of damage thresholds may be possible.

The investigations have also shown that LAYERTEC’s optimized silver mirrors possess significant LIDT values in the fs range. Another advantage of silver mirrors is their extremely broad zero-GDD reflectance band with reflectance up to 98.5 % at normal incidence. Even silver mirrors with a defined transmission of 0.01 % exhibit considerable damage thresholds, especially with respect to dielectric ultra broadband components. For more information on silver mirrors see page Front Surface Silver Mirrors (400 – 4000 nm).

References

[1] B. Mangote, L. Gallais, M. Commandré, M. Mende, L. O. Jensen, H. Ehlers, M. Jupé, D. Ristau, A. Melninkaitis, J. Mirauskas, V. Sirutkaitis, S. Kicas, T. Tolenis, R. Drazdys: “Femto-second laser damage resistance of oxide and mixture oxide optical coatings”; Optics Letters 9 (Vol. 37), p. 1478-1480 (2012)

[2] Raluca A. Negres, Christopher J. Stolz, Kyle R. P. Kafka, Enam A. Chowdhury, Matt Kirchner, Kevin Shea, Meaghan Daly: “40-fs broadband low dispersion mirror thin film damage competition”; Proceedings Volume 10014, Laser-Induced Damage in Optical Materials 2016, 100140E (2016)

Overview about Laser-Induced Damage Thresholds of Ultrafast Laser Optics

Coating	Reflectance at 800 nm	LIDT	Pulse Duration, Repetition Rate
Single-wavelength AR coating**	< 0.2 %	1.10 J/cm2 3) 1.20 J/cm2 2)	42 fs, 1 kHz 1 ps, 1 kHz
Broadband AR coating**	< 0.5 %	1.20 J/cm2 2)	1 ps, 1 kHz
Unprotected gold	97.5 %	0.33 J/cm² 1) 0.33 J/cm2 2)	50 fs, 1 kHz 150 fs, 1 kHz
fs-optimized silver	98.5 %	0.38 J/cm2 1) 0.38 J/cm2 2)	50 fs, 1 kHz 150 fs, 1 kHz
Enhanced silver (600 – 1200 nm)	98.5 %	0.24 J/cm2 2)	150 fs, 1 kHz
Partially transparent silver (T = 0.01 % @ 800 nm)	98.5 %	0.22 J/cm2 2)	150 fs, 1 kHz
High power mirror for fs pulses	> 99.5 %	0.90 J/cm2 3) 3.60 J/cm2 4)	42 fs, 1 kHz 70 fs, 10 Hz
Enhanced silver (800 nm)	99.7 %	0.37 J/cm2 2)	150 fs, 1 kHz
Negative-dispersion mirrors*	> 99.9 %	0.10 J/cm2 2)	150 fs, 1 kHz
Broadband low-GDD mirrors*	> 99.9 %	0.15 J/cm2 1) 0.10 J/cm2 2)	6 fs, 4 kHz 150 fs, 1 kHz
Standard low-GDD mirrors	> 99.9 %	0.50 J/cm2 3) 2.40 J/cm2 4) 0.30 J/cm2 2) 0.55 J/cm2 2)	42 fs, 1 kHz 70 fs, 10 Hz 150 fs, 1 kHz 1 ps, 1 kHz
High power mirror for ps pulses	> 99.9 %	0.35 J/cm2 1) 0.44 J/cm2 2) 0.65 J/cm2 2)	50 fs, 1 kHz 150 fs, 1 kHz 1 ps, 1 kHz
1) Measurements were performed at Friedrich Schiller University Jena
2) Measurements were performed at Laser Zentrum Hannover
3) Measurements were performed at Wigner Research Centre for Physics, Budapest
4) Measurements were performed at Helmholtz-Zentrum Dresden-Rossendorf
* A significant number of designs were tested. The LIDT values stated here are typical for the corresponding test conditions.
** Self-focusing effects may destroy the substrate while the AR coating is still intact.

Metallic High Power Mirrors

GDD of High Power Ultrafast Laser Mirrors