Optics Performance at High-Power Levels

SPIE Photonics West conference in San Jose, January 2008

Ola Blomster, Magnus Pålsson, Sven-Olov Roos, Mats Blomqvist,

Felix Abt, Friedrich Dausinger, Christoph Deininger, Martin Huonker

OPTOSKAND ● FGSW ● TGSW ● TRUMPF

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ABSTRACT

High laser power levels combined with increasing beam quality bring optics performance into focus. The subject of optics performance is a hot topic, but lack of a common nomenclature, as well as of proper measurements, makes the situation confusing. This paper will introduce a nomenclature for comparing the performance of different types of optics. Further, the paper will present a test setup for characterizing optics, along with test results for different optics materials and designs. The main influence of high power levels on optics is a focal shift along the optical axis. In industrial applications, this might influence the performance of the process, especially if the focal shift is in the range of the Rayleigh length. In the test setup that is to be presented, the optics are exposed to a high power beam, and a pilot beam is used for measuring the change in focal position. For a proper description of optics performance, the laser beam parameters should not influence the measured results. In the nomenclature that will be presented, the performance is related to the Rayleigh length for a fundamental mode beam. The performance of optics when used with multimode beams will be presented.

Keywords: high power lasers, focal shift, thermal lensing, lens material, anti-reflection coating, FSF, SFSF

1 INTRODUCTION High power lasers are today frequently used in various industrial applications, such as welding and cutting. As the average laser power increases and the beam quality improves, there is an increasing interest in optics performance. Optical components can act differently from low power applications to high power applications. The reasons for the variations are many and there are no standard for defining and compare optical systems. This report will focus on setting a definition for the focal shifts and also introduce results from optical systems with respect to this definition.

The lens material and coating of lenses are assumed to be parameters that have an impact on the focal shifts, since most changes are believed to be of thermal nature. Previous studies1-4 have shown that lenses exposed to high optical powers will, to different extent, experience changes in focal length. However, these studies do not give a complete picture, which includes the dependence on the focal shift of all parameters of interest. Also, the presentation of focal shifts is different in each study, which makes the results difficult to compare.

1.1 Thermal Lensing

When a laser beam passes through an optical material some absorption will occur. The absorbed power will be converted to heat and conducted to the surface of the material, inducing a temperature gradient in the material. The acquired temperature gradient depends on the power density of the laser light, the amount of absorption in the material, and it is inversely proportional to the thermal conductivity of the material. Due to the temperature dependence of the refractive index, the temperature gradient will result in a refractive index gradient throughout the material. This phenomenon of a thermally induced refractive index gradient in an optical material is referred to as thermal lensing due to the fact that the material will behave as a lens. The temperature gradient will also result in other alterations of the material properties; the non-uniform thermal expansion may change the radius of curvature of the thermal lens and stress induced by the changing temperature in the center of the material will further change the radius of curvature of the thermal lens – the elasto-optic effect.

1.2 Lens Materials, Coatings and Surface Contamination

Different materials contribute in various degrees to thermal lensing. For high-power laser applications it is most common to use various qualities of fused silica, which generally have a low coefficient of thermal expansion, high transmission from ultraviolet to infrared light, and scratch resistant surfaces. An alternative is to use Gradium, where the lenses consist of several axial gradient glass layers, for which the refractive index varies. These layers are combined into a single spherical lens with a continuous refractive index. This will reduce the spherical aberrations, but also increase the sensitivity to thermal effects.

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To reduce reflections from lens surfaces dielectric multi-layer anti-reflection coatings are commonly applied. The number of layers, the coating materials and the deposition techniques all affect the thermal effect on the coating.5

If an optical surface is slightly contaminated, heat absorption will increase. A surface with particulate contamination in thermal contact will have non-uniform absorption, concentrated at the contamination spots. This will result in local maxima in the temperature gradient, inducing power intensity disturbances, creating hot spots in the focus position. Molecular contamination on the other hand usually forms in a uniform layer, with absorption that can be a significant factor for thermal lensing.6

2 DEFINITIONS AND NOMENCLATURE In this section definitions and nomenclature for focal shift in high power optics will be introduced.

2.1 Focusing of a Fundamental Mode Gaussian Beam

Fig. 1. Definitions of parameters for focusing of fundamental mode Gaussian beams.

The beam radius, w, for a Gaussian beam is defined as the radius at which the irradiance has decreased to 13.5% (1/e2) of the central part of the beam. This is identical to the radius within which 86.5% of the power is falling. The beam radius along the optical axis (z-axis) is given by

( )220 Θ⋅+= zww , (1)

where w, w0, z and Θ are given in Fig. 1. When measuring the angle Θ, the definition of the beam radius is the same as the definition of the beam radius w. The waist radius w0 is given by

Θ⋅

=π

λ0w , (2)

where λ is the wavelength. The focusing angle Θ is approximately given by

f

D⋅

=Θ2

, (3)

where D is diameter of the beam and f the focal length. The Beam Parameter Product, which is a measure of the quality of the beam, is defined as

πλ=Θ⋅= 0wBPPFM . (4)

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As noted we can see that the Beam Parameter Product is independent on the focus geometry. The index “FM” indicates “Fundamental Mode”, which gives us the theoretical limit. The Rayleigh length, ZR, is defined as the distance from the beam waist, where the beam radius has increased by a factor of 2 . For the fundamental mode (indicated by the index “FM”) we get

20

Θ⋅=

Θ=

πλw

Z RFM . (5)

2.2 Focusing of Multimode Beams

For multimode beams, the definition of beam size and other beam parameters are not as well defined as for the Gaussian beam. The calculated values are now more to be seen as approximations. The beam radius, r, is defined as the radius within which 86.5% of the radiation is falling. The beam size along the optical axis is given by

( )220 Θ⋅+= zrr . (6)

The beam radius at the waist, r0, is given by

Θ⋅⋅=⋅=

πλ2

02

0MwMr , (7)

where M2 is the “Beam Propagation Factor”. Accordingly the Beam Parameter Product for the multimode beam (indicated by the subscript “MM”) is given by

FMMM BPPMBPP ⋅= 2 . (8)

The Rayleigh length for the multimode beam is also scaled with the M2 factor, and is given by

RFMRMM ZMZ ⋅= 2 . (9)

2.3 Focal Shift

The focal shift introduced in an optic system/element caused by high power, is the shift in the z-direction (along the optical axis). The focal shift can be measured either by a measurement of the focal shift directly on the high power beam, or by introducing a pilot beam through the optics, to monitor the focal shift. To be able to compare different optics and systems we introduce normalizations towards the Rayleigh length.

Fig. 2. Definitions of focal shift in an optic system.

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2.3.1 Characterizing the Optics

In cases where we are interested in characterizing the optics, we normalize the focal shift towards the Rayleigh length of a fundamental mode beam with the same size as the high power beam, focused free of aberration, with a lens having the same focal length as the measured object. This normalization we call the focal shift factor.

The Focal Shift Factor (FSF) is defined by

RFM

FS

ZZ

FSFΔ

= , (10)

where ΔZFS is the measured focal shift, and ZRFM is the Rayleigh length for a fundamental mode beam (Gaussian beam) with the same diameter as the high power beam, focused by a diffraction limited optics with the same focal length as the measured optics. ΔZFS > 0 indicates an elongation of the focal length, as shown in Fig. 2.

2.3.2 Characterizing a complete System

To characterize a complete system, also the lens aberrations and the beam quality of the laser used has to be taken into consideration. To normalize the focal shift, we now use the real Rayleigh length given by the high power beam. It can be estimated by the formulas in Sec. 2.2, but is preferably measured. The System Focal Shift Factor (SFSF) is defined by

RMM

FS

ZZ

SFSFΔ

= . (11)

It has to be noted that the SFSF only describes the influence of focal shift, but to get a complete image of the optics performance also the BPP has to be given.

2.4 Interpretation of Focal Shift Factor and System Focal Shift Factor

In the focusing of a fundamental mode beam, the introduced focal shift gives rise to an increased spot size according to

20 1 FSFww += . (12)

A focal shift factor of 1 indicates a change in the spot size by approximately 40%. As rule of thumb this is the maximum change acceptable for an application. Of course this has to be judged due to the application.

In the case of a multimode beam the focal shift gives rise to an increased spot size according to

20

2

20 11 SFSFrMFSFrr +=⎟

⎠⎞

⎜⎝⎛+= . (13)

An FSF of M2, or an SFSF of 1 indicates a change in the spot size by approximately 40%.

It is also important to note that this description of the FSF and SFSF does not include astigmatism aberrations.

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3 EXPERIMENTAL The purpose of the experiments was to investigate some of the parameters that affect the thermally induced focal shift in optics caused by exposure of a high power laser beam. Using a pilot laser in addition to the high power laser, and letting both beams pass through the optics in test performed the experiments. The measurement beam incidents normal to the test object, whereas the high power beam was aligned to pass through the test object at a small angle in order to separate the two beams after the test object and direct the pilot beam to a camera. The camera scans the beam path to find the focus position of the beam at different power levels.

3.1 Experimental Setup

Fig. 3. Shows a schematic description of the experimental setup.

A schematic description of the experimental setup is shown in Fig. 3. The high power laser used in this experiment was an IPG YLR-4000-SS, with 4 kW maximum output power at approximately λ = 1070 nm. The output fiber was connected via a fiber-to-fiber optic switch, a feeding fiber, a fiber-to-fiber coupling unit, and a process fiber to a collimating unit. The pilot laser was a fiber-coupled diode laser emitting at λ = 845 nm with a maximum output power of 5 mW. The collimating units were either diameter 25 mm units with focal length f = 60 mm or diameter 50 mm units with f = 160 mm. The high power beam is deflected slightly more than 90° using a high reflectivity mirror (99.6% at λ = 1070 nm). The mirror was positioned so that the high power had an incident angle of 1-4° when hitting the test objects, the angle depending on the focal length of the test objects. The pilot beam was deflected 90° by a mirror and then transmitted through the mirror for the high power beam. To separate the pilot beam from the high power beam and direct it into the camera a beam splitting mirror was used. Since the high-power beam was aligned at an angle it passed next to the mirror to hit a thermal head, whereas the pilot beam was deflected 90° and directed into the camera.

The camera used for the measurements was a WinCamD-UCD12 equipped with a ½” CCD sensor with 4.65 × 4.65 μm2 pixel size. On the camera an imaging lens system, consisting of an f = 40 mm collimating lens and an f = 80 mm focusing lens, was mounted. The camera was mounted on a Physik Instrumente (PI) M-126.DG1 motorized linear stage with very high position precision. The camera and the linear stage were controlled through a LabView program.

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3.2 Measurement Procedure

As described in the previous section, the high power beam and the pilot beam passes through the test object with a slight difference in angle. The Gaussian pilot beam was chosen to have normal incidence, since measuring on a beam perpendicular to the test object would give more accurate results. The high power beam was then adjusted so that the beam would hit the test object with its center coinciding with the center of the test object, but with a small angle of incident. After alignment, two types of measurements were performed. In the first measurement the aim was to investigate what happens during the first few minutes after the laser was turned on. The focal shift measurements were then performed when the test object had stabilized and entered steady state.

4 RESULTS In this section results will be presented for both single lens elements as well as for complete optical systems.

4.1 Thermal Shift in single Lens Elements

Fig. 4. Shows the beam radius versus time during the first three minutes, when the laser power is increased from 2 to 4 kW

for a diameter 25 mm lens of focal length f = 83 mm. The beam radius is measured at approximately two Rayleigh lengths behind focus.

A typical transient behavior for a lens (diameter 25 mm, focal length f = 83 mm) with relatively large focal shift is shown in Fig. 4. In the measurement, the camera was positioned approximately two Rayleigh lengths behind focus. The major change in focal position occurs during the first 40 seconds, then the focal shift saturates.

In Fig. 5. the beam radius as a function of camera position is shown for the same lens at steady state for optical power levels; 0 kW, 2 kW and 4 kW. For this lens the FSF was calculated to 9.9 at 2 kW and to 20.6 at 4 kW using Eq. (10), where λ = 1070 nm and D is the 1/e2 diameter of the incident high power beam, which is estimated to 18 mm, in Eqs. (5) and (3), respectively.

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Fig. 5. Shows the beam radius versus camera position for laser power 0, 2 and 4 kW for a diameter 25 mm lens of focal

length f = 83 mm. Measurements were performed after the system had reached steady state.

4.1.1 Lens Materials

The lens material has a decisive effect on the FSF. In our experiments we have compared various qualities of fused silica (singlet, doublet and triplet lenses) as well as lenses made of Gradium. We note that for the same focal length and lens diameter, focal shift factors vary from almost 0 to more than 20 at 4 kW output optical power. As shown in Table 1., there is also a variation for lenses within the same batch.

4.1.2 Coatings

In a measurement series different coatings of the same lens type was compared. As can be noted in Table 1., there is a prononced difference in FSF between the two different coating techniques. In one case the coating was a 3-layer e-beam deposition (SiO2/HfO2), and in the other case the coating was a 5-layer magnetron sputtered deposition (SiO2/HfO2).

Table 1. Sample of tested lenses.

Diameter [mm]

Focal length [mm]

Lens type Coating ZRFM [µm]

FSF (2 kW) FSF (4 kW)

25 100 Singlet, lens 1 Coating 1 42.1 -1.62 -4.35

25 100 Singlet, lens 2 Coating 1 42.1 -0.05 -0.95

25 100 Singlet, lens 3 Coating 1 42.1 -0.74 -1.38

25 100 Singlet, lens 1 Coating 2 42.1 -4.00 -10.49

25 100 Singlet, lens 2 Coating 2 42.1 -4.28 -11.89

25 100 Singlet, lens 3 Coating 2 42.1 -5.97 -14.34

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4.2 Thermal Shift in optical Systems

-2,5

-2

-1,5

-1

-0,5

0

0 2000 4000 6000 8000 10000 12000 14000 16000

Laser Power [W]

Ther

mal

Shi

ft [m

m]

-1,25

-1

-0,75

-0,5

-0,25

0

SFSF

Fig. 6. The thermal shift of a TRUMPF-Laser CFO50 optics system with a collimating focal length of 200 mm and a

working focal length of 280 mm. Measurements were performed after the system had reached thermal equilibrium.

-8

-7

-6

-5

-4

-3

-2

-1

0

0 1000 2000 3000 4000 5000 6000 7000 8000

Laser Power [W]

Ther

mal

Shi

ft [m

m]

-0,8

-0,7

-0,6

-0,5

-0,4

-0,3

-0,2

-0,1

0

SFSF

Fig. 7. The thermal shift of a TRUMPF-Laser PFO 3D scanner optics system with a collimating focal length of 138 mm and

a working focal length of 450 mm. Measurements were performed after the system had reached thermal equilibrium.

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Measurements of optical systems have been performed with different lasers and optics. Fig. 6. shows the thermal shift for a Trumpf CFO50 optics with a collimating focal length of 200 mm and a working focal length of 280 mm. Two TruDisk 8002 lasers were used and the focal shifts were measured with a Primes FocusMonitor. All components were new and cleaned, and the Rayleigh length was assumed to be constant over the whole power range. The focal shift exhibits an almost linear dependence on the laser power. A similar behavior is observed for a TRUMPF-Laser PFO 3D scanner optics system with a collimating focal length of 138 mm and a working focal length of 450 mm, as shown in Fig. 7. In this case the laser was an HLD 8002.

-5,0

-4,0

-3,0

-2,0

-1,0

0,0

0 100 200 300 400 500 600 700 800 900

Laser Power [W]

SFSF

f160QD/f200Gf160QD/f200QDf160QD/f400QS

Fig. 8. System Focal Shift Factor for three different optical systems. All systems had the same f = 160 mm quartz doublet

collimating lens, whereas the focusing lenses were shifted. Measurements were performed after the system had reached thermal equilibrium. For M2 values, relate to Fig. 9.

In another experiment the System Focal Shift Factor was measured for various optical systems using an IPG YLR-1000-SM single-mode fiber laser and a Primes HighPower-MicroSpotMonitor system. Having the same collimating lens (an f = 160 mm quartz doublet) and changing the focusing lens, Fig. 8. was obtained. We note that the SFSF is considerable larger using an f = 200 mm Gradium focusing lens compared to an f = 200 mm quartz doublet focusing lens and an f = 400 mm quartz singlet focusing lens, whereas the difference between the quartz lenses is small.

The increase in SFSF as a function of laser power seems to be linear up to approximately 600 W. At higher power values it seems to fall of. This behavior is explained in Fig. 9., where the M2 values of the laser and optics in combination are plotted together with the optical system including the Gradium focusing lens. As can be noted, the M2 values starts to increase at about 500 W. For the SFSF the M2 are included, see Eqs. (9) and (11), thus one have understand that both thermal shift and beam quality of the laser system influence the final beam focus at the output side. One should also note that the power distribution of the focused beam was not taken into account in the calculations of the SFSF.

10

-5,0

-4,0

-3,0

-2,0

-1,0

0,0

0 100 200 300 400 500 600 700 800 900

Laser Power [W]

SFSF

0,0

0,5

1,0

1,5

2,0

2,5

M 2

Fig. 9. System Focal Shift Factor for an optics system with an f = 160 mm quartz doublet collimating lens and an f = 200 mm Gradium focusing lens. Also shown are the M2 values for the laser and optics in combination.

5 CONCLUSIONS A new nomenclature for comparing the performance of optical elements as well as of complete optical systems has been introduced. The performance of the optics is related to the Rayleigh length for a fundamental mode beam, where the Focal Shift Factor (FSF) is defined as the measured focal shift divided by the Rayleigh length for the fundamental mode beam. In characterizing a complete optical system, the System Focal Shift Factor (SFSF) is defined as the measured focal shift divided by the real Rayleigh length given by the high power beam, and which includes the M2 value for the laser and optics in combination. An experimental setup for characterizing single lens elements using a pilot laser in addition to the high power laser was described. Results for single lens elements showed FSF values from 0 to more than 20 at 4 kW output optical power. Measurements of SFSF have been performed with different lasers and optical systems - up to almost 1 kW using a single-mode fiber laser and up to 16 kW using a multi-mode disc laser.

REFERENCES

1 O. Märten, R. Kramer, H. Schwede, V. Brandl, and S. Wolf, (2007), “Determination of Thermal Focus Drift with High Power Disk and Fiber Lasers”, Proc. of the 4th Int.l WLT-Conference on Lasers in Manufacturing (2007).

2 F. Abt, A. Heß, and F. Dausinger, “Focusing of high power single mode laser beams”, ICALEO 202 (2007). 3 D. F. de Lange, J. T. Hofman, and J. Meijer, “Optical characteristics of Nd:YAG optics and distortions at high power”, Pre-print

for Proceedings of ICALEO (2005). 4 J. D. Mansell, J. Hennawi, E. K. Gustafson, M. M. Fejer, R. L. Byer, D. Clubley, S. Yoshida, and D. H. Reitze, “Evaluating the

effect of transmissive optic thermal lensing on laser beam quality with a Shack–Hartmann wave-front sensor”, Appl. Opt. 40(3), 366-374 (2001).

5 R. S. Shah, J. J. Rey, and A. F. Stewart, “Limits of Performance - CW Laser Damage”, Proc. of SPIE 6403, 640305 (2007). 6 C. A. Klein, “Figures of Merit for High-Energy Laser-Window Materials: Thermal Lensing and Thermal Stresses”, Proc. of SPIE

6403, 640308 (2007).

Article authors:

Ola Blomster*, Magnus Pålsson, Sven-Olov Roos, Mats Blomqvist Contribution by: Karin Laag and Frida Nero

Optoskand AB Krokslätts Fabriker 27, SE-431 37 Mölndal, Sweden

* ola.blomster@optoskand.se; phone +46 31 706 27 63; fax +46 31 706 27 78; www.optoskand.se

Felix Abt, Friedrich Dausinger,

Forschungsgesellschaft für Strahlwerkzeuge mbH Pfaffenwaldring 43, D-70569 Stuttgart, Germany;

Christoph Deininger,

Technologiegesellschaft für Strahlwerkzeuge mbH Rotebühlstrasse 87, D-70178 Stuttgart, Germany;

Martin Huonker

TRUMPF Laser GmbH & Co. KG Aichhalder Strasse 39, D-78713 Schramberg, Germany