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Abstract
The Laser Interferometer Space Antenna (LISA) will be the first space-based gravitational wave observatory. LISA uses continuous-wave, infrared laser beams propagating among three widely separated spacecrafts to measure their distances with picometer accuracy via time-delay interferometry. These measurements put very high demands on the laser wavefront and are thus very sensitive to any deposits on laser optics that could be induced by laser-induced molecular contamination (LIMC). In this work, we describe the results of an extensive experimental test campaign assessing LIMC related risks for LISA. We find that the LIMC concern for LISA, even considering the high demands on the laser wavefront, may be greatly reduced compared to that observed at shorter wavelengths or with pulsed laser radiation. This result is very promising for LISA as well as for other space missions using continuous-wave, infrared laser radiation, e.g., in free space laser communication or quantum key distribution.
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1. INTRODUCTION
The laser technique is becoming increasingly important for space applications. Challenging missions use or plan to use lasers for different variants of light detection and ranging (LIDAR), e.g., to detect water vapor and aerosols [1,2] or methane [3] from space. Although LIDAR can also be performed from ground or airplanes, only space missions have the capability to provide data with global coverage. As a recent milestone, the European Space Agency (ESA) placed the wind LIDAR mission Aeolus into space in 2018, which for the first time demonstrated the space use of a high-power pulsed laser at UV wavelength [4–6]. Lasers are also used in planetary exploration to perform laser-induced breakdown spectroscopy (LIBS) on Mars and the Moon [7–9]. Another major field is free space laser communication, where the demand to transfer a large amount of data over long distances drives the design towards increasing laser power [10]. Additional applications such as solar harvesting [11], in-space active debris removal [12], and laser propulsion [13,14] have been studied for a long time but have recently regained commercial interest and might boost the future usage of lasers in space.
The long-term operation of lasers in space brings many challenges, and optical components have to satisfy stringent requirements concerning precision and reliability [15,16]. A critical conundrum for operating lasers in space is laser-induced molecular contamination (LIMC) [17–19]. LIMC denotes the interaction of laser radiation with volatile molecules on optical surfaces leading to the formation of deposits. Sources of contamination are organic materials and silicones, e.g., adhesives, insulation material, or printed circuit boards. Although outgassing can be reduced, e.g., by selecting low-outgassing materials (which is a requirement according to international space standards [20,21]) or bake-out above the operational temperature, LIMC is difficult to prevent totally. A laser-induced deposit can trigger laser-induced damage [22–25] and also negatively affect the performance (e.g., laser transmission/reflection, wavefront, or scattering) of a laser optics. LIMC has been found to be a critical topic in many fields—in particular if lasers are used in a vacuum environment—such as fundamental science [26] and lithography [27,28], and for optical laser components such as beam expanders [29]. In fact, we frequently find laser-induced deposits when inspecting our own laboratory laser systems, e.g., on laser frequency conversion crystals, or on the entrance/exit windows of vacuum spatial filters (vacuum tubes with a pinhole for mode cleaning). For space applications, such performance losses are particularly threatening, because optical components typically can neither be cleaned nor replaced. Since space missions exhibit high costs and effort, such risks need to be mitigated carefully. It is also advisable to assess LIMC related risks early in the project, since they can otherwise lead to heavy delays. For example, the Aeolus mission required a careful screening of materials with LIMC tests as well as a redesign of the laser head, which had to be pressurized with oxygen to mitigate deposit formation [18,30,31]. Although this mission was very successful in the end, the mission start was delayed by 10 years with respect to original time plans.
Fig. 1. Gravitational wave spectrum showing detectors and known sources of gravitational waves. Reprinted with permission from NASA Goddard Space Flight Center [33].
This paper is concerned with the assessment of LIMC related risks for a specific scientific space mission called the Laser Interferometer Space Antenna (LISA) [32]. LISA aims at detecting gravitational waves in the 0.1 mHz to 1 Hz frequency band and is a flagship space mission with an expected launch in the mid to late 2030s. Gravitational waves in this range are expected to originate from different sources (see Fig. 1)—some of them may even still be unknown—but cannot be measured with terrestrial interferometers [e.g., the Laser Interferometer Gravitational-Wave Observatory (LIGO)] due to seismic noise and interferometer arm length limitations [34].
LISA uses time-delay interferometry [35] to measure the distance variation (induced by a gravitational wave) using laser beams propagating among three widely spaced spacecrafts (separated by 2.5 million km). One-link measurements will be recombined with appropriate delays to form three (non-independent) Michelson interferometers. Due to this highly sensitive measurement, LISA performance is expected to be very sensitive even to deposits with nanometer thickness. We have thus conducted research to assess whether a laser-induced deposit can be generated with laser parameters as intended to be used on the free space optics of the optical bench (OB) and telescope of LISA.
The remaining paper is organized as follows: in Section 2, we explain why even very thin (nanometer thickness) deposits would already adversely affect the gravitational wave detection performance of LISA. We then report the results of our experimental test campaign consisting of a series of LIMC tests with different test parameters (parametric test, Section 3) as well as a long-duration LIMC test (Section 4) with a test duration of 6 months. Fortunately, none of these tests showed the formation of a laser-induced deposit. In Section 5, we discuss this finding with a simple kinetic model for laser-driven deposit formation. Finally, we draw conclusions on the LIMC related risks for LISA and other missions with similar laser parameters.
2. WHY LIMC MIGHT BE CRITICAL FOR THE LISA SPACE MISSION
LISA uses continuous-wave, 1064 nm laser systems using Nd:YAG non-planar ring oscillators (NPROs). These laser sources need to provide an output power of 2 W and stabilization of frequency noise to better than $ \sim 20 \;{\rm Hz}/\sqrt {{\rm Hz}}$ between $\sim {5}\;{\rm mHz}$ and 1 Hz and relative power noise below $2 \times {10^{- 4}}/\sqrt {{\rm Hz}}$. Although the detailed laser design has not yet been fixed, it is expected that these will be entirely fiber based [34,36], meaning that LIMC is not expected to occur within the laser system itself. However, there are free space optics on LISAs moving optical subassemblies (MOSAs), which consist of an OB and a telescope and can be fine-pointed to the location of their associated remote spacecraft [37]. These optics are high-reflection (HR) laser optics and will most likely have ion beam sputtered coatings with a silicon dioxide (${{\rm SiO}_2}$) top layer, since these have been found to be favorable for space applications [38,39]. The maximum laser power density on these optics was calculated to be below $150\; {{\rm W/cm}^2}$.
In terms of LIMC, a long wavelength of 1064 nm (corresponding to photon energy of 1.16 eV) in combination with rather low power density should greatly reduce the risk of a deposit formation compared to lasers operating in the UV or with high peak power densities as encountered for pulsed lasers. The reason for this is that LIMC is (particularly in early stages of deposit formation, where there is a negligible absorption of laser power by the deposit) driven by photo-chemistry [40]. A typical chemical bond of an organic molecule (e.g., 3.6 eV for a C-C chemical bond) requires either UV photons or multi-photon absorption for photo-chemical bond dissociation. This will be discussed in more detail in Section 5. On the other hand, LISA targets a long mission duration (4.5 years with a planned extension of 6 years) and cumulative long-term effects need to be studied carefully. Furthermore, a laser-induced deposit on laser optics can induce minute changes to the wavefront.
This has been demonstrated with a simple laboratory experiment at the DLR Laser Optics Test Center for Aerospace Applications; see Fig. 2. Wavefront measurements were performed by first transmitting a He–Ne laser beam through a pristine position (meaning without a laser-induced deposit) of the laser optics. Subsequently, the optics were moved with a translation stage to place the deposit into the center of the beam to perform the actual wavefront measurement.
In this experiment, we used laser optics with a laser-induced deposit generated in a previous nanosecond pulsed LIMC test using laser radiation at 266 nm [42]. This deposit had a height of approximately 40 nm and the typical “donut” shape, which is frequently encountered in laser-induced deposits, which has recently been attributed to originate from diffusion of adsorbates on the optical surface [43].
Interestingly, we observe a similar shape when measuring the wavefront of a He–Ne laser beam transmitted through the deposit using a Shack–Hartmann (SH) wavefront sensor. This effect relates to the change in the optical path length of the laser radiation transmitted at the location of the deposit.
As already mentioned, the LISA mission has very high demands on the laser wavefront to achieve the targeted picometer accuracy in interferometric distance measurements as required for the detection of gravitational waves. The reasons are twofold. First, wavefront modifications will generate more scattering and loss of transmitted power to the distant spacecraft with a corresponding increase of the shot noise limit. The second—and more important—effect is that a perturbed wavefront couples to the spacecraft jitter and will thus generate variations in the measured path lengths. This effect has been modeled mathematically [44,45] and is qualitatively explained in Fig. 3. If spacecraft 1 (SC 1) emits a spherical wavefront, a jitter of its attitude will not affect the measured distance at SC 2 at 2.5 million km distance [see Fig. 3(A)]. As opposed to this, a distorted wavefront [Fig. 3(B)] will mean that even a small pointing jitter (a requirement for the LISA mission is $8 \times {10^{- 9}}\;{\rm rad}/\sqrt {{\rm Hz}}$ [46]) will lead to an apparent distance that changes with the attitude of SC 1.
Fig. 2. Demonstration experiment showing the changes to the wavefront of a laser induced by a laser-induced deposit. (A) Schematic of the experimental setup. (B) Deposit morphology as measured with white-light interference microscopy (WLIM). Right panel shows a cut along $y$ axis at $y = {1}\;{\rm mm}$. (C) Wavefront detected at the Shack–Hartmann wavefront sensor. Left panel: three-dimensional plot of the measured wavefront. Right panel: two-dimensional plot with the same color scale as the left panel. Figure adapted from Ref. [41].
An estimate from the LISA contamination working group is that a wavefront deformation of the order of 2 nm per optical surface would already start adversely affecting the gravitational wave detection performances. Such a wavefront distortion could be introduced even by a very thin laser-induced deposit.
3. PARAMETERIC LIMC TEST
A. Parametric LIMC Test Setup
Figure 4 describes the experimental setup used for the parametric LIMC test campaign.
Figure 4(A) provides an overview of the setup. 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 ${ \lt 10^{- 9}} \;{\rm mbar}$ using a turbo-molecular pump backed by a scroll pump. The chamber has large entrance and exit ports, which allows for simultaneously irradiating anti-reflection (AR) as well as HR optics.
Figure 4(B) shows a picture of the optical beam line at the entrance of the vacuum chamber. A 10 W, continuous-wave, 1064 nm fiber laser (IPG Photonics Inc., YLR-10-1064-LP) is placed below the optical beamline (bottom of the picture). The first $\lambda /2$-wave plate and the polarizer are used to attenuate the power of the beam. The second $\lambda /2$-wave plate is used to change the power ratio between the laser beams exiting the polarizing beam splitter (PBS) cube, which then enter the vacuum chamber and are directed to the sample holder. The 1064 nm beams are focused with an ${f} = {500}\;{\rm mm}$ lens, giving a near-Gaussian beam profile with a width of 1.9 mm at the optics under test.
Figure 4(C) shows a picture of the sample holder and contamination source inside the vacuum chamber. The sample holder is made from stainless steel and carries the AR optics (0° angle of incidence for the laser beam) and HR optics (20° angle of incidence) under test. A ${{\rm CaF}_2}$ witness sample (Korth Kristalle GmbH) for contamination control via Fourier-transform infrared (FT-IR) spectroscopy is mounted to a third position [47]. A high-resolution linear actuator (Physikalische Instrumente GmbH, M-230.25) is used to vertically translate the sample holder inside the vacuum chamber. This allows for irradiation of different positions on the optical samples. Molecular contamination is introduced by heating a mixture of outgassing materials inside the contamination source. This source is made from copper and has three effusion holes with 10 mm diameter placed at a distance of approximately 35 mm from the optics under test. A heat shield from stainless steel reduces the radiative heat transfer from the contamination source (temperatures up to 100°C) to the sample holder (25°C).
To detect a possible deposit formation during the experiment, we measured the transmission of the AR optics as well as the reflection from the HR optics with three power detectors (Ophir Optronics Solutions Ltd, PD300) measuring the reflexes from fused silica wedges. Furthermore, the polarization of the HR beam was detected with a commercial rotating-wave plate polarimeter (Thorlabs Inc., PAX1000IR1). Finally, we also measured the wavefront of a frequency-stabilized He–Ne laser (Newport Corp., Spectra Physics 177 A, 633 nm wavelength), which was spatially overlapped with the 1064 nm laser beam incident on the AR optics. The reason for performing a wavefront measurement at 633 nm (instead of directly measuring the wavefront at 1064 nm) was that wavefront measurements with our SH wavefront sensors (Thorlabs Inc., WFS30) were found to be more repeatable and had a lower measurement error. Since there is a fundamental trade-off between wavefront resolution and wavefront lateral resolution in wavefront measurements with a SH sensor [48], we used two SH sensors with a different pixel pitch for wavefront detection. One sensor (SH-A) used a $2\times$ beam expansion and a 300 µm pixel pitch micro-lens array (MLA) with a specified resolution of 3.2 nm ($\lambda /200$) with the goal of detecting thin deposits with a large lateral dimension. The second sensor (SH-B) used a ${4\times}$ beam expansion and a 150 µm pixel pitch MLA (with a specified resolution of 3.6 nm ($\lambda /100$) to detect deposits/wavefront changes with small lateral dimensions. The He–Ne laser was used with a very low power density and was transmitted through the vacuum chamber only to perform wavefront measurements to ensure that it did not generate a laser-induced deposit itself. Experimental details for the execution of the LIMC tests as well as the optics and materials under test are provided in Appendix A. An important experimental detail is that the laser wavefront measurements were performed after a “plane wave” calibration on a pristine position of the optical samples (similar to the experiment described in Section 2 and Fig. 2), which were not irradiated with the 1064 nm fiber laser. This scheme for data acquisition, which requires a movement of the sample holder, optical shutters, as well as calibration and wavefront measurement with the SH wavefront sensors, was performed automatically with a software written in LabVIEW. The shutters ensure that the 1064 nm fiber laser is blocked during the wavefront measurements, since it would otherwise oversaturate the wavefront sensors. Similarly, the He–Ne laser (although used at a very low power density of ${\lt}1\;{{\rm mW/cm}^2}$) was blocked during irradiation with the 1064 nm fiber laser. This reduces the time of irradiation with the He–Ne laser to less than 1 min per wavefront measurement and less than 1 h per LIMC test. Furthermore, it avoids any possible deposit formation driven by simultaneous two-color irradiation.
Fig. 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 (A) spherical wavefront centered around SC1 and (B) distorted wavefront. The distorted wavefront will introduce a strong dependence of the distance measured between the spacecrafts on the attitude of SC1 (pointing jitter). Such wavefront distortion can, for example, be introduced by a laser-induced deposit. Credit: ICSO 2023 [41].
B. Parametric LIMC Test Results
Table 1 provides an overview of the LIMC tests and their test parameters performed within the parametric test campaign.
Initially, we performed a bake-out of the vacuum chamber at temperatures up to 120°C and performed a LIMC blank test with a laser fluence of $150\;{{\rm W/cm}^2}$ to ensure the cleanliness of the chamber. Subsequently, we executed a series of LIMC tests with a fluence of $150\;{{\rm W/cm}^2}$, a test duration of 120 h, and an increasing temperature of the contamination source to enhance the outgassing rate of the contaminant (25°C, 70°C, and 100°C for tests A, B, and C, respectively). Furthermore, an additional test with an extended test duration of 240 h at 70°C and a higher laser fluence ($300\;{{\rm W/cm}^2}$) was performed. All tests were done following the ISO technical report ISO/TR 20811:2017 [49].
Figure 5 shows the results of the in situ measurements during the parametric test. The most important result is that all LIMC tests with a maximum outgassing temperature of 70°C showed no significant changes of transmission, reflection, wavefront, or polarization. This indicates that no deposit was generated during these tests. This was also verified via an inspection of all tested laser optics with differential interference contrast (DIC) and fluorescence microscopy (Olympus, BX61) with $200\times$ magnification. Fluorescence microscopy was performed with a 100 W mercury-vapor light source, but optical filters led to a monochromatic excitation at 375 nm. Fluorescence in the visible was detected with a XM10 black and white camera. The micrographs were processed with an image algorithm for contrast enhancement. From our experience with other LIMC tests, this means that no deposits with a thickness above 5 nm were generated, since these can be safely detected.
At the highest source temperature of 100°C (test C), we observed strong losses of transmission (AR coated optics) and reflection (HR coated optics). A microscopic inspection of the tested optics revealed that these changes are not induced by a laser-induced deposit, but can rather be attributed to condensation. An analysis of the ${{\rm CaF}_2}$ witness sample from LIMC test C via FT-IR spectroscopy revealed that condensation products include hydrocarbons as well as esters (see Appendix B). Figure 6 shows the Normarski image of optics from LIMC test C showing droplets from condensation as well as the result of a corresponding wavefront measurement. This condensation is caused by the large temperature difference between the contaminant (100°C) and the tested optics (25°C). Interestingly, the change in the wavefront could be observed only with SH sensor B (optimized for a high lateral resolution). This can be explained by the small lateral size of the condensation droplets, which had a typical diameter of only 10 µm. The root-mean square (RMS) of the wavefront change integrated over the detection area of the wavefront sensor increased by up to 60 nm [see Fig. 5(D)]. Fortunately, such a condensation effect is unlikely to occur during the LISA space mission, since large temperature differences between an outgassing material and laser optics are not expected.
Fig. 4. (A) Schematic of the experimental setup for the parametric LIMC test. EM-CCD, electron-multiplying CCD camera; WP, wave plate; HR, high-reflection coated optics; AR, anti-reflection coated optics; ${{\rm MOC = CaF}_2}$, witness sample; BP, bandpass filter. (B) Picture of the optical beamline in front of the vacuum chamber. (C) Picture of the contamination source and the sample holder inside the vacuum chamber.
Table 1. Overview of Test Parameters in the Parametric LIMC Test Campaign
4. LONG-DURATION LIMC TEST
A. Long-Duration LIMC Test Setup
Since the parametric LIMC test did not show any formation of a laser-induced deposit, we decided to modify the LIMC test setup prior to starting a long-duration test (see Fig. 7). The goal of this modification was to increase the sensitivity for detecting very thin laser-induced deposits with our in situ detection methods (transmission, polarization, wavefront) by superimposing changes of these parameters coming from the irradiation of eight optics under test. We thus built a new sample holder carrying four HR and four AR coated laser optics, which is depicted in Fig. 8. The 1064 nm beam of the 10 W fiber laser was incident on all of these laser optics. The idea behind this approach is that thin laser-induced deposits might be detected if LIMC occurs on several optical surfaces simultaneously and leads to an increased accumulated change of laser polarization, wavefront, or transmission. To ensure that outgassing molecules reach all optical surfaces of the optics under test, the contamination source from the parametric test was used without the lid. Furthermore, the source was re-positioned below the sample holder. To use the new sample holder, it also became necessary to modify the optical beam line. Instead of being focused into the vacuum chamber (as for the parametric test), the 1064 nm laser beam of the IPG Photonic fiber laser was reduced by a factor of 2.5 and collimated to a beam diameter of 2.2 mm giving a laser fluence of $150\;{{\rm W/cm}^2}$ (for 0° angle of incidence) for all tested optics. For this setup, it was not possible to use the beam of the He–Ne laser for wavefront detection, since the transmission/reflection of the AR/HR optics (with coatings optimized for 1064 nm) were too low at the wavelength of the He–Ne laser (632 nm). Wavefront sensing was thus achieved with a frequency-stabilized diode laser operating at 1064 nm (NP Photonics, “The Rock”). The beam of this low power laser (1 mW) was spatially overlapped with the beam of the IPG Photonics laser using crossed polarizations and a PBS. Wavefront detection at 1064 nm also required purchasing and integrating a new wavefront sensor (Optocraft SHSLab HR2-130-GE-PRO) into the setup. An important experimental detail of the long-duration LIMC test was that the setup was connected to an uninterrupted power supply to ensure that the test was not interrupted in case of a power shortage.
Fig. 5. Results of in situ measurements during the LIMC tests. (A) Transmission of the anti-reflection coated optics. (B) Reflection of the high-reflection coated optics. (C) Pressure in the vacuum chamber as measured with an ion gauge. (D) Root-mean square (RMS) of the wavefront change as measured with Shack–Hartmann sensor B (with ${4\times}$ beam expansion and 150 µm MLA). (E) Degree of polarization measured in the beam reflected from the HR optics. Note that LIMC test D continued until a total irradiation time of 240 h without significant changes in the measured parameters (data not shown).
Fig. 6. Condensation during test C of the parameteric LIMC test. Left panel: DIC micrograph of the AR optics (front surface) showing droplets due to condensation. Right panel: example of a wavefront measurement with Shack–Hartmann sensor B near the end of the LIMC test of the same optics.
Fig. 7. Schematic of the experimental setup for the long-duration LIMC test. EM-CCD, electron-multiplying CCD camera; WP, wave plate; BP, bandpass filter; PBS, polarizing beam splitter.
B. Results of the Long-Duration LIMC Test
The long-duration LIMC ran successfully for the entire test duration of 6 months. During this time, it was necessary only to exchange a turbo-molecular pump, which could be done without interrupting the test. Figure 9 shows the results of the in situ measurements. The test was performed with a set of nine outgassing materials (see Appendix A) and a temperature of 30°C, which is at the upper limit of the nominal temperature intended for the operation of the LISA laser and OB. [50] Similar to the long-duration LIMC tests, the optics under test had a temperature of approximately 25°C. The test had a contaminant mass loss of 0.24%. It is found that the transmission [Fig. 9(A)] as well as the wavefront [Fig. 9(D)] were very stable (changes ${\lt}{1}\%$) over the entire test duration of 180 days. The polarization [Fig. 9(B)] was also stable, except for a small (2%) change after an irradiation time of 125 days, after which the degree of polarization returned to its original value after 143 days. This change is an experimental artifact caused by a slight change in the alignment due to the mechanical movement of the sample holder. We thus conclude that the in situ measurements indicate a negative LIMC test, meaning that no laser-induced deposit was generated. This result was also confirmed via microscopic inspection (Nomarski and fluorescence microscopy at ${200} \times$ magnification), which was performed on all eight tested laser optics. We found no indication of laser-induced deposits and no laser-induced damage.
Fig. 9. Results of the long-duration LIMC test. (A) Transmission of the optics under test (four HR and four AR optics). (B) Degree of polarization measured with the polarimeter. (C) Pressure in the vacuum chamber as measured with an ion gauge. (D) Root-mean square (RMS) of the wavefront change as measured with Shack–Hartmann sensor.
5. DISCUSSION
In summary, we have performed a parametric test campaign as well as a long-duration test to assess possible LIMC related risks for the LISA mission. The parametric test used five different contaminant materials and a laser power density of up to $300\;{{\rm W/cm}^2}$. To accelerate LIMC testing, the temperature of the contamination source was increased up to 100°C. At 100°C, we observed condensation on the front surface of the optics under test kept at 25°C, but no LIMC was observed in the remaining tests with outgassing temperatures of up to 70°C. The long-duration LIMC test used a set of 10 contaminant materials (outgassing temperature 30°C) with a laser power density of $150\;{{\rm W/cm}^2}$. This test used an improved setup to measure accumulated changes of transmission, wavefront, or polarization coming from eight optics under test. This test also showed no indication of LIMC in any of the in situ (transmission, polarization, wavefront) or ex situ (Nomarski and fluorescence microscopy) measurements. From our experience with pulsed LIMC tests, deposits containing hydrocarbons or silicones (unless fully oxidized to ${{\rm SiO}_2}$) are typically visible via fluorescence and Nomarski microscopy once a height of 5 nm is reached. This should be considered as an upper limit for the sensitivity for deposit detection in all of our LIMC tests. For the long-duration LIMC test, we have irradiated a set of eight optical samples, and the laser beam was incident on 12 optical surfaces (from AR and HR laser optics). Nonetheless, the wavefront was stable within $\pm {20}\;{\rm nm}$. This corresponds to a wavefront change of ${20}\;{\rm nm/12} = {1.7}\;{\rm nm}$ per optical surface, which agrees with the LISA requirement of 2 nm per optical surface.
To further assess LIMC related risks for LISA, it is also useful to discuss this topic in terms of a kinetic model. As discussed in Section 2, photon energy of 1.16 eV (at the LISA wavelength of 1064 nm) is much lower than typical bond energies of organic molecules (e.g., 3.6 eV for a C-C chemical bond). This means that photo-chemical bond breaking requires multi-photon absorption (at least three-photon absorption), and thus LIMC formation should be strongly dependent on the peak power density. In fact, this is also evident from other experiments/lasers operating at a wavelength near 1 µm; see Table 2.
Table 2. Laser Parameters and Calculated Reaction Rates for Laser Systems Operating near 1 µm Wavelengtha
The LISA space mission will use continuous-wave lasers, with an average power density ${P_{{\rm av}}}$ of $150\;{{\rm W/cm}^2}$ on free space optics, where our LIMC test campaign has not revealed any evidence for deposit formation. Interestingly, the ground based gravitational wave interferometer “Advanced LIGO” (aLIGO) uses optical surfaces with a much higher power density ${P_{{\rm av}}}$ of up to $200\;{{\rm kW/cm}^2}$ on some optics inside the interferometer. Nonetheless, a detailed report on the optical contamination control (concerning hydrocarbon and particulate contaminants) does not mention any problems due to organic molecular contamination. It is even reported that an organic spray is used to clean dust from laser optics [51].
As opposed to this, nanosecond pulsed LIMC tests (with a peak power density of $570\;{{\rm MW/cm}^2}$, e.g., in tests for the Atmospheric Laser Doppler Instrument (ALADIN) of the ESA Aeolus mission, have shown the growth of organic molecular contamination. In previous work [18], seven contaminant materials were tested in LIMC tests. Whereas no contamination was observed at an outgassing temperature of 40°C, two materials, namely, Solithane 113 (Uniroyal Chemical Company Inc.) and EC-2216 B/A (3M Scotch-Weld), showed LIMC at an elevated outgassing temperature of 100°C (as opposed to the LIMC tests from this work, the tests at 100°C from Ref. [18] were performed in a sealed vacuum chamber without continuous pumping). These two materials have thus been included in the LIMC tests of this activity. Another molecule known to generate LIMC with nanosecond pulsed lasers near 1 µm wavelength is toluene [52].
Serious problems due to LIMC have also been reported for the Allegra laser system at the Extreme Light Infrastructure (ELI) beamlines operating at 1030 nm wavelength [26]. It was found that laser pulses before pulse compression (with a pulse duration $\tau$ of 0.5 ns, giving a peak power density of ${P_{{\rm peak}}} = 570\;{{\rm MW/cm}^2}$) did not generate visible deposits. As opposed to this, pulses with the same laser fluence ($F = 260\;{{\rm mJ/cm}^2}$) generated strong deposits on laser optics after pulse compression ($\tau = 3\;{\rm ps}$, ${P_{{\rm peak}}} = 87\;{{\rm GW/cm}^2}$).
This strong dependence of LIMC formation on peak power density is also illustrated in Fig. 10.
Fig. 10. Dependence of LIMC on the peak power density for different lasers operating with a wavelength near 1 µm.
In an attempt to model this observation, we assume that LIMC formation at a wavelength of 1 µm is a reaction driven by three-photon absorption and that this absorption is the rate limiting step of the photo-chemical reaction. This assumption agrees well as with the observation that LIMC at 355 nm (3.49 eV) scales linearly with the laser fluence (for the contaminant EC2216 with a nanosecond pulsed LIMC test at outgassing temperatures of 40°C and 60°C) [53]. In this case, the reaction rate $r$ would be linearly proportional to the third power of the peak power density ${P_{{\rm peak}}}$. [54] For pulsed lasers, it furthermore has to be considered that the reaction will occur only during the fractional time of power emission (${t_{{\rm on}}}/({t_{{\rm on}}} + {t_{{\rm off}}})$), which can be expressed as the product of the pulse duration $\tau$ and the pulse repetition rate ${f_{{\rm rep}}}$:
The reaction rates (normalized to one for the LISA parameters) calculated for the different application scenarios are provided in Table 2. We find that the reaction rate for LIMC formation for LISA would be by a factor of $2 \cdot {10^9}$ below the reaction rate for aLIGO (where there is no observation of LIMC) and by a factor of $2 \cdot {10^{13}}$ below the test conditions used for the ALADIN instrument. This means that a deposit formed with a reaction rate expected for aLIGO build up within 1 s would need approximately 60 years to be built up with the reaction rate estimated for LISA. This might explain why no LIMC can be observed in our LIMC test. Certainly, this model has many simplifications. For example, the energy and thus bond energies of molecules adsorbed on surfaces or surface defects can significantly deviate from gas phase energies. In fact, our model does not include any modeling of the optical surface. Particularly, if a laser-induced deposit starts to build up, it may absorb the laser radiation, which can start to thermally accelerate the deposit formation and change its chemical structure [40]. Additionally, there is a high number of outgassing molecules for each contaminant material, which can be analyzed via gas chromatography mass spectrometry (GC-MS) [53]. Each of these molecules can have a different reaction mechanism and thus a different kinetics for deposit formation. Nonetheless, it is clear that the laser parameters of LISA (low photon energy and low peak power density) are very favorable to mitigate LIMC.
Table 3. List of Materials Used for LIMC Testing
Table 4. Optics Used for LIMC Tests
6. CONCLUSIONS AND OUTLOOK
The experimental results as well as theoretical considerations from this work indicate that LIMC might be less of a concern for the LISA mission (even considering the high demands on the laser wavefront) compared to other space missions operating with pulsed laser radiation or at shorter wavelengths. Neither the parametric LIMC test nor the long-duration LIMC test showed any indication of a deposit formation. This is encouraging for LISA as well as for other space missions using continuous-wave, IR laser radiation with low power density (up to $150\;{{\rm W/cm}^2}$). Of course, this statement should be taken with care, because
- (1) the LISA mission targets a long mission duration (4.5 years $+$ a possible 6 year extension), whereas the test duration was only up to 6 months;
- (2) the LISA mission is a flagship project with a high mission budget (${\gt}{1}$ billion Euros);
- (3) the LISA mission will use many contaminant materials not covered by the test campaign.
We thus suggest that additional LIMC tests should be performed during the assembly and integration phase of the LISA mission to cover all potential outgassing materials.
APPENDIX A: TESTED MATERIALS AND OPTICS
Table 3 shows a list of contaminant materials used for LIMC testing in the different work packages of this activity.
All tests were performed with a material mixture, and materials were tested with a mass of 1 g each. Materials were selected either because they have been shown to lead to LIMC in nanosecond pulsed LIMC tests at 1064 nm [18] or because their usage is planned for the LISA mission. Note that more materials have been added throughout the test campaign.
Table 4 provides a list of laser optics used for LIMC testing. All coatings had a top layer of silicon dioxide. The parametric LIMC test used electron beam coated optics, whereas the long-duration test was performed with optics coated via ion beam sputtering.
The rationale behind this is that porous e-beam coatings have been found to be more susceptible to LIMC in previous LIMC test campaigns and thus represent a worst case test scenario. As opposed to this, the long-duration LIMC test was performed with dense ion-beam sputtered coatings that are representative of future flight optics.
APPENDIX B: ANALYSIS OF THE FT-IR SPECTRA OF WITNESS SAMPLES
${{\rm CaF}_2}$ witness samples from the LIMC tests have been analyzed with FT-IR spectroscopy. Only the analysis of the witness sample from test C showed detectable peaks in the FT-IR spectrum indicating contamination.
Table 5 provides the outcome of the analysis of contamination levels for different contaminant classes following ECSS-Q-ST-70-05C Rev.1 [47].
Table 5. Contamination Levels for the ${\rm CaF}_2$ Witness Sample of LIMC Test C
The analysis reveals that hydrocarbons as well as esters condensed on the surface of the witness sample.
Funding
European Space Agency (4000131933/20/NL/IB/gg).
Acknowledgment
DLR and SpaceTech GmbH acknowledge funding from ESA, molecular contamination de-risking activities for LISA–EXPRO+.” The authors acknowledge the Glasgow LISA team (University of Glasgow) as well as the NASA Goddard Space Flight center for providing samples of contaminant materials and guidance for their selection.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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