Mirrors for Superior Industrial Compressors
By Darius Silinskis, 2020

High-power ultrashort pulse industrial lasers
Since the invention of CPA [1], a method of generating highintensity ultrashort optical pulses, it has evolved to become
one of the most dominant techniques [2]. Further advances ofsolid-state and fiber technologies have enabled both compact
and less complex high-power ultrashort pulse lasers to be built,
in order to address the industrial market. Material processing
applications like glass cutting or the dicing of silicon wafers haverevealed a further demand for the advancement of ultrashortlasers. These lasers need to meet the following main industrial
requirements: they must be robust, as well as having a stablepulse to pulse energy and beam profile. Moreover, the abovementioned features need to be ensured in serial production. Allof this has dictated the requirement for optical components tobe used to build such industrial lasers.

Ultrashort pulse,
low energy

Chirped pulse,
low energy

Seedoscillator

StretcherM

Chirped pulse,
high energy
AmplifierGGCompressor

Ultrashort pulse,
high energyFigure 1. Schematic of the CPA system.

Challenges for high power compressors in serial production
Even though there are many variations of compressors, mostof them have the same main construction (Figure 1). Thereare components to compress the pulse, such as diffraction
gratings, prism pairs, etc., and there are mirrors to reflect thebeam. While the LIDT of diffraction gratings is considered to bea limiting factor for high power compressors [3], this limitation
could be bypassed by choosing bigger gratings. However, thisincrease means that the size of the mirrors in the compressor
should also increase, which results into challenges for keeping a

low reflected beam distortion. Keeping in mind that mirrors facemultiple reflections, the requirement for mirrors flatness starts
to play a key role. But having flat mirrors is not enough, as theseshould also be well aligned. This is directly related with theangular precision of mirrors, and in the case of retroreflecting
roof mirrors, it results in right angle and pyramidal errors. Flatand precisely angled mirrors reduce the time needed for theassembly of compressors, which is one of the key targets ofserial production.

Altechna’s solutions
Low beam distortion
0.036 λ0.035 λ0.03
Peak0.02

0.02
0.01

00

Peak–0.01
–0.02

–0.02
–0.03
–0.04

Valley–0.05–0.053 λ–0.04

PV 0.089λ
RMS 0.013λPower -0.011λ
IRR: B 0.081λValley–0.057 λPV 0.092λ
RMS 0.014λPower -0.007λ
IRR: B 0.085λFigure 2. Measured flatness values before the coating (left)
and after the coating with stress compensation (right), area
95 mm x 20 mm.

The stress induced by mirror coatings is one of the main reasons
why even very flat substrates get deformed after coating, andthis eventually has an impact on the distortion of the beamprofile [4]. There are several ways to overcome this issue; forexample, by having thick substrates or by employing lessstress-inducing coating technologies. However, these solutions
have disadvantages, especially when lasers are being built
for industrial applications where a small footprint of the laser
is desired. As a result of all this, Altechna has developed atechnique to compensate for the stress introduced by thick
sputtered coatings. This technique allows the flatness after
coating to be comparable to the flatness of the substrate before
coating (Figure 2), and it could even be applied with relatively
thin mirrors. Moreover, by using sputtering technology, themirrors reach >99.99% reflectivity and the optimization of thegroup delay dispersion minimizes the impact of a pulse chirp,
which eventually reduces the complexity of the compressor
design calculations (Figure 3).

Group Delay Dispersion s-pol

Group Delay Dispersion p-pol

1010
GDD

7.5
52.5
GDD (fs 2)

GDD (fs 2)

Std Dev

52.5
0-2.50-2.5-5-5

-7.5

-7.5

-10GDD

7.5

Std Dev

960

980 1000 1020 1040 1060 1080 1100 1120 1140 1160 1180

-10960980 1000 1020 1040 1060 1080 1100 1120 1140 1160 1180

Wavelength (nm)Wavelength (nm)Figure 3. GDD measured in reflection for s and p polarizations.

Fast alignment
Although the alignment of the diffraction gratings or prisms
in the compressors might be considered as the biggest
challenge [5], the precision of the mirror angles should also
be taken into account. Mirrors with imprecise right angles will
introduce additional complexity for the alignment, which inserial production equates with lower productivity. With this inmind, Altechna offers either free-standing mirrors with precise
angles or mirror assemblies – so-called retroreflecting roof
mirrors (Figure 4). Depending on the size of the mirrors, these
could reach a +/-5 arcsec right angle and +/-1 arcmin for thepyramidal error. Controlling these two parameters reduces thetime needed for the alignment while building the compressors.

Figure 4. HR Retroreflectors.

Environmental stability

typically appears as a spectral shift. This effect is mainly causedby the water absorption of the porous coatings, which eventuallychanges the refractive index [6]. To overcome this issue, Altechna
is making mirrors with a sputtering technology that guarantees
a bulk-like layer density which leads to environmental stability.
The computer controlled process ensures repeatability and a big
coating area that is well-suited for serial production (Figure 5).

Environmental stability is one of the key requirements forindustrial grade lasers and systems. Stability needs to be ensured,
even when environment conditions like the temperature orrelative humidity are changing. It should also be noted that for
laser components like mirrors, the impact of the environment

Vacuum chamber
Substrate holder
8RF plasma
sourceSputter source 1
Turn table65

9RF plasma source
Ar plasma
FS substrate
Rotating substrate holder

7104

1132

1213

11415

Sputter source 3

Loadingchamber
Sputter source 2

Figure 5. Altechna’s Magnetron Sputtering coating technology.

References[1] D. Strickland and G. Mourou, Opt. Commun., 56, 3, 219 (1985).
[2] https://www.laserfocusworld.com/lasers-sources/article/14035380/femtosecond-lasers-from-inception-to-industrial-applications[3] N. Bonod and J. Neauport, Diffraction gratings: from principles to applications in high-intensity lasers, Vol. 8, No. 1, Advances inOptics and Photonics.
[4] T. Tolenis, L. Grinevičiūtė, L. Mažulė, A. Selskis and R. Drazdys, „Low-stress phase plates produced by serial bideposition of TiO2 thinfilms”, J. Nanophoton. 10(3), 036003 (2016), doi: 10.1117/1.JNP.10.036003.
[5] I. V. Yakovlev, Stretchers and compressors for ultra-high power laser systems, Quantum Electronics 44 (5) 393 – 414 (2014).
[6] J. R. Gee, I. J. Hodgkinson, H. A. Macleod, Moisture-dependent Anisotropic Effects in Optical Coatings. Applied Optics 1985, 24,3188–3192.
www.phototechnica.co.jp

Altechna, UAB〒336-0017 埼玉県さいたま市南区南浦和 1-2-17

TEL:048-871-0067 FAX:048-871-0068

e-mail:voc@phototechnica.co.jp

info@altechna.com
www.altechna.com