2.1

Laser-induced contamination in space lasers

LIC has been a critical conundrum for several space missions. In particular, the ESA mission Aeolus for measuring global wind profiles with a wind LIDAR using nanosecond-pulsed laser radiation at 355 nm, has only become possible after extensive ground-based test campaigns to find suitable materials. Furthermore, the laser head had to be redesigned for operating in a partial oxygen atmosphere instead of in vacuum [2, 3, 4]. LIC has also been proposed to be a contributor to severe performance losses on the LITE instrument and the GLAS laser on Icesat [5].

LIC is known to be particularly prominent for lasers operating in the ultraviolet (with a high photon energy that allows for bond breaking) and for short laser pulses (which trigger multi-photon absorption). The LISA space mission uses lasers with a wavelength of 1064 nm and continuous wave laser radiation, which significantly reduces the risk of LIC.

On the other hand, the LISA space mission can be expected to be sensitive even to very thin deposits affecting the wave front, consequently degrading gravitational wave detection performance. This will be explained in the following subsection. Furthermore, LISA targets a long mission duration (4.5 years with a planned extension of six years) and cumulative long-term effects need to be studied carefully to minimize their impact for this flagship mission.

2.2

Why LIC is critical for the LISA space mission

For any laser-system, LIC can lead to a loss of performance or failure. In particular, a deposit on a laser optic can absorb laser-radiation. Transmission or reflection loss aside, the absorption of energy can also trigger laser-induced damage [6, 7]. An effect that has not been of concern for most missions so far, is that a laser-induced deposit induces minute changes to the laser-wavefront.

This effect has been showcased in a simple laboratory experiment, at DLR premises. We have measured the change of the wavefront of a Helium-Neon laser when passing through an optical component bearing a laser-induced deposit, see Figure 2. The deposit on the laser-optic had been generated in a previous LIC test with a nano-second pulsed laser at 266 nm [8]. The deposit (on the front side of the optics) had a height of~40 nm with the typical “donut”-shape [8]. A similar shape was observed in the wavefront measurement of the Helium-Neon laser beam transmitted through the deposit. This effect relates to the change in the optical path length of the laser radiation transmitted at the location of the deposit.

Figure 2:

Wavefront changes induced by a laser-induced deposit. A) Experimental setup for detecting the laser-induced wavefront. B) Deposit morphology as measured with white-light interference microscopy (WLIM). C) Wavefront detected at the Shack-Hartmann wavefront sensor.

The LISA mission has very high demands on the laser wavefront quality, in order to achieve interferometric distance measurements with picometer accuracy as required for the gravitational wave detection. The reasons are twofold, Firstly, the wavefront variations coupled with a S/C pointing jitter will create path length variations. Additionally, wavefront modifications will generate more scattering and loss of transmitted power to the distant S/C with a corresponding increase of the shot noise limit. The first-most important-effect was simulated mathematically and reported in literature [10, 11], and is qualitatively explained in Figure 3. In panel A of Figure 3, a laser is emitted from spacecraft 1 (SC 1) and forms a spherical wavefront (centered around SC 1) when reaching spacecraft 2 (SC 2), at a target distance of 2.5 million km. In this case, a small pointing jitter (a requirement for the LISA mission is 8 × 10^–9rad Hz^–1/²) will not affect the measured distance. As opposed to this, a distorted wavefront will lead to an apparent distance that changes with the attitude of SC1 (see panel B of Figure 3). The LISA contamination working group has estimated that wavefront deformation of order of 2 nm per optical surface would start adversely affecting the gravitational waves detection performances. Of course, such small wavefront change could easily be introduced even by a very thin (nm level) laser-induced deposit.

Figure 3:

Explanation for the coupling of the wavefront to the jitter (attitude variation) of spacecraft 1 (SC 1) to the apparent distance measured at spacecraft 2 (SC 2) for a spherical wavefront centered around SC1 (panel A) and a distorted wavefront (panel B). The distorted wavefront will introduce a stong dependence of the distance measured between the spacecrafts on the attitute of SC1 (pointing jitter). Such a wavefront distortion can for example be introduced by a laser-induced deposit.

3.1

Experimental setup

Figure 4 shows a schematic of the experimental setup used for LIC testing. The main part is a vacuum chamber composed of standard CF components with copper sealings, a chamber volume of ~30 liters and a base pressure of <10^-8 mbar obtained with using a turbo-molecular pump backed by a scroll pump.

Figure 4:

LIC Setup for LISA contamination testing. EM-CCD = electron-multiplying CCD camera, WP=wave plate, HR = high-reflection coated optics, AR=anti-reflection coated optics, MOC=CaF2 witness sample.

The sample holder made from stainless steel is located in the centre of the LIC chamber and has one position for mounting a high-reflective sample (~20 degrees angle of incidence) and one position for mounting an anti-reflective sample (0° angle of incidence), see Figure 5. Additionally, a CaF₂ witness sample sample (Korth Kristalle GmbH) for contamination control via Fourier-transform infrared (FT-IR) spectroscopy [12] is mounted to a third position. The temperature of the sample holder can be monitored with a thermocouple and a thermal button logger (tempmate.®-B5). The sample holder can be translated along the z-direction with a high-resolution linear actuator (Physikalische Instrumente GmbH, M-230.25).

Figure 5:

Contamination source and optics under test.

Molecular contamination is introduced by a contamination source, which is made from copper and designed as an effusion cell with three holes of 10 mm diameter. For LIC testing, we used a distance of ~35 mm between the holes of the effusion cell and the laser optics. The source was heated via heating cartridges and the temperature was monitored with a thermocouple. A heat shield made from stainless steel reduced the radiative heat transfer from the source to the sample holder. By additionally wrapping parts of the contamination source with Aluminium foil (without covering the emission holes), it was possible to heat the contaminant up to 120°C, while keeping the sample holder at temperatures between 20°C and 25°C.

For LIC testing, the optics under test were irradiated with a continuous-wave, 1064 nm fibre laser (IPG Photonics Inc., YLR-10-1064-LP). The output of that laser was split into two beams to simultaneously irradiate the AR optics and the HR optics under test.

We used several in-situ measurements for detecting a possible deposit formation during the LIC test. In particular, the transmission of the AR optics and the reflection of the HR optics were measured with power detectors (Ophir PD300) probing reflections of the laser beam from fused silica wedges (see Figure 4). Furthermore, we measured the polarization of light reflected from the HR optics with a polarimeter (Thorlabs Inc. PAX1000IR1). Finally, we measured changes to the wavefront of a frequency stabilized HeNe laser (Spectra Physics 117A, 633 nm wavelength) that was spatially overlapped with the beam of the 1064 nm fiber laser. The spatial overlap of the two lasers on the front surface of the AR optics was controlled by detecting scattered light with a long-distance microscope and an electron-multiplying CCD camera. The reason for detecting the wavefront of an overlapped HeNe laser, instead of directly detecting the wavefront of the 1064 nm laser, was that wavefront measurements with the HeNe laser and our Shack-Hartmann wavefront sensors (WFS30, Thorlabs Inc.) were more repeatable and had a lower RMS measurement error. We used two Shack-Hartmann sensors (WFS30, Thorlabs Inc.) for wavefront detection, since there is a fundamental trade-off between wavefront resolution and wavefront lateral resolution in wavefront measurements with a Shack-Hartmann (SH) sensor [13]. The beam directed to the first SH-sensor (SH-A) was used with a 2x beam expansion and a 300 μm pixel pitch micro-lens array (MLA) with a specified resolution of 3.2 nm (λ/200). The second SH-sensor (SH-B) was used with a 4x beam expansion and uses a 150 μm pixel pitch MLA (with a specified resolution of 3.6 nm (λ/100)) to detect wavefront changes with small lateral dimension. Note that the He-Ne laser was not maintained on during most of the testing (as this too could have been a source of LIC) but only at short intervals to obtain the wavefront measurement.

3.2

Tested materials and laser optics

Table 1 provides a list of contamination materials selected for the LIC test campaign. All materials (~1g each) were cured at room temperature (curing duration >7 days) on UHV compatible Aluminum foil. Afterwards, these materials were cut into small pieces to improve their homogeneity in the contamination source. Materials have been selected either because they were found to generate laser-induced deposits in previous LIC tests with nanosecond pulsed laser irradiation at 1064 nm [14] or because their usage is planned for the LISA mission.

Table 1.

List of tested contamination materials.

The AR optics used for the LIC tests were electron-beam anti-reflection coated (AR/AR1064+532 @0° AOI), 1-inch diameter windows (fused silica substrate: PW1004UV, Laser Components GmbH) and were tested at an angle of incidence (AOI) of 0°. The HR optics used for the LIC tests were electron-beam high-reflection coated (HR 750 - 1100 nm @ 0-45°), 1-inch diameter mirrors (fused silica substrate: PW1004UV, Thorlabs Inc.) tested at an AOI of 22.5°.

3.3

Test procedure

LIC tests have been performed following the ISO technical report ISO/TR 20811 [12]. In particular, the laser optics were inspected before and after LIC testing via differential interference contrast (DIC) and fluorescence microscopy. Deviating from the ISO technical report, we have not used in-situ fluorescence detection, as presence thereof, at the LISA wavelength, is not expected to occur. Instead, in-situ measurements for wavefront and polarization have been implemented.

The procedure for wavefront detection during the LIC test included the following steps: First, the irradiation with the 10 W fiber laser was blocked. Then the sample holder was translated by 5 millimetres to a calibration position. Then the shutter of the HeNe laser was opened and the wavefront sensors were calibrated (“plane wave calibration”). The sample holder was then moved back to its original position and the wavefront was measured. This way comparison of the transmitted wavefront on a pristine position on the optical sample to a position with a possible laser-induced deposit generated by the 1064 nm laser is obtained. This procedure means that there is only a short time (~30 seconds) between the calibration and the measurement with the wavefront sensors, which minimizes effects due to long-term sensor drifts or wavefront changes of the HeNe laser. For this procedure it is important that the HeNe laser does not generate LIC itself. This is ensured by using a short irradiation time (~15 seconds / wavefront measurement) and a low laser power density, as already mentioned. The described measurement procedure was performed automatically with a self-programmed software written in LabVIEW that controlled the laser beam shutters, the wavefront sensors and the translational stage of the sample holder.

4.1

Overview and in-situ test results

Table 2 provides an overview of the LIC tests described in this publication. Initially, we performed a bake-out of the vacuum chamber at temperatures up to 120°C and performed an LIC blank test with a laser fluence of 150 W/cm² to ensure the cleanliness of the chamber.

Table 2.

Overview of LIC test parameters.

Subsequently, we performed a series of LIC tests with a fluence of 150 W/cm² and an increasing temperature of the contamination source to enhance the outgassing rate of the contaminant (Tests A, B and C). At the highest source temperature of 100 °C (Test C) we observed strong losses of transmission (tested for the AR coated optics) and reflection (tested for the HR coated optics), see Figure 6. From a microscopic inspection, this loss of transmission/reflection could clearly be attributed to a condensation of outgassing material on the front surface of the tested laser optics (see Figure 8). This is a result of the high temperature difference between the contamination source (100°C) and the laser optics under test 25°C.

Figure 6:

Results of in-situ measurements during the LIC tests. Panel A: Transmission of the anti-reflection coated optics. Panel B: Reflection of the high-reflection coated optics. Panel: C: Pressure in the vacuum chamber as measured with an ion gauge. Panel D: Root-mean square (RMS) of the wavefront change as measured with Shack-Hartmann sensor B (with 4x beam expansion and a 150 μm MLA). Panel E: Degree of polarization measured in the beam reflected from the HR optics. Note that LIC Test D has continued until a total irradiation time of 240 hours without significant changes in the measured parameters (data not shown).

In an attempt to accelerate the LIC test by other means, we have decided to perform another LIC test (Test D) with a temperature of 70°C and a higher laser fluence (300 W/cm²). Furthermore, the test duration was increased to 240 hours. Similar to LIC tests A and B, no deposit formation and no condensation was observed.

4.2

Changes in the transmitted wavefront due to condensation

The condensation observed in LIC test C also affected the wavefront of the transmitted laser beam, see Figure 7. Interestingly, this effect could only be observed with the Shack-Hartmann sensor B (with the higher lateral resolution). The change in wavefront was as high as ±150 nm. The root-mean-square (RMS) of the wavefront change integrated over the detection area of the wavefront sensor is given in panel D of Figure 6 and increased up to ~60 nm. For the other LIC tests, the RMS of the wavefront change was below 10 nm.

Figure 7:

Change of wavefront during LIC Test D as measured with Shack-Hartmann sensor B. The changes in the wavefront are introduced by condensation of the front surface of the AR laser optics.

4.3

Microscopic inspection

All laser optics tested within this campaign have been analyzed with DIC and fluorescence microscopy at 2000x magnification. Except for the LIC test with condensation, we found no indication of a laser-induced deposit. From our experience, this means that no LIC with a height above ~5 nm was generated (detection limit).

Figure 8 shows a DIC micrograph of the front surface of the AR optics of LIC Test C showing the condensation products. The condensation occurs as droplets with a diameter of ~10μm. This is much smaller than the lateral resolution of the wavefront sensor even considering the 4 times beam expansion. This indicates that the wavefront change is introduced by inhomogeneities in the density of these droplets rather than due to an imaging of individual droplets.

Figure 8:

DIC micrograph of the AR optics (front surface) of Test C showing a strong condensation of outgassing material.