2.1 LID threshold test for optical components used in Regen cavity

Damage thresholds of optical components used in Regen cavity were tested by a home-made laser damage test system with real-time damage event imaging and detection^{[Reference Wang, Wei, Jiang, Wang, Qiao, Guo, Fan and Li15]}. The test followed by a standard 1-on-1 testing regime. The test results are shown in Table 1, in which damage thresholds of all optical components in Regen cavity are larger than $2.5~\text{J}/\text{cm}^{2}$ . Since the peak energy fluence in the Regen cavity is about $1~\text{J}/\text{cm}^{2}$ , it can be inferred that LID is not the ultimate cause of damages on surfaces of optical components in the Regen.

Table 1. Damage threshold of optical components used in the Nd:glass Regen.

2.2 Observation of damage morphology by an optical microscope

Morphology of damage sites can be observed by an optical microscope (KH-8700, HIROX-USA, Inc) with high magnification. The KH-8700 microscope contains an integrated stepping motor which allows for fast, smooth, and accurate scanning with $0.05~\unicode[STIX]{x03BC}\text{m}$ per pulse precision and 30 mm of automated travel distance, which makes 3D model imaging and 3D height measurement available. In Figures 2 and 3, the morphology of damage spots of a quarter waveplate (QWP) and a window glass (WG) of the Pockels cell (PC) in the Regen is shown with $1000\times$ and $350\times$ magnification, respectively. It can be seen from these figures that there are some deposits on damage sites compared to undamaged sites. And the thickness of deposited material on the QWP and WG is 4.67 and $6.16~\unicode[STIX]{x03BC}\text{m}$ , respectively, which can be read from Figures 2(b) and 3(b). Figure 4 shows the morphology of the whole damage sites on the WG surface of PC in the Regen. From the figure, we can see that deposits only exist in the region on which the laser pulse irradiates, which is a typical characteristic of LIC^{[Reference Riede, Schroeder, Bataviciute, Wernham, Tighe, Pettazzi and Alves14]}. There are less and smaller deposits on the central parts of this region. The reason for this may be that the spatial center of laser beam has the highest energy fluence, which has a clean effect for the deposits and results in removal of the center part of deposits where laser intensity is highest at the beginning of their growth. Moreover, as a comparison, the morphology of a damage spot on an Nd:glass rod surface caused by LID when laser energy fluence exceeds its damage threshold in the LID test experiment mentioned in Section 2.1 is shown in Figure 5. It is obvious that on the damage site the coating was peeled off and no deposits were observed, though after damage the surface shape of the Nd:glass rod had distorted severely.

Figure 2. Morphology of a damage site on a QWP surface in the Regen cavity. (a) Optical microscopy of the QWP damage site with $1000\times$ magnification; (b) contour map of the QWP damage site.

Figure 3. Morphology of a damage site on a WG surface of the PC. (a) Optical microscopy of the WG damage site with $350\times$ magnification; (b) contour map of the WG damage site.

Figure 4. Morphology of the whole damage sites of WG surface of PC in the Regen.

Figure 5. Morphology of a damage site on an Nd:glass rod surface caused by LID in the LID test experiment when laser energy fluence exceeds the damage threshold of the Nd:glass rod surface.

Figure 6. (a) Raman spectra of deposits on the damaged QWP surface in comparison to that of carbon; (b) Raman spectra of deposits on the surface of damaged WG of PC in comparison to that of carbon.

2.3 Chemical analysis of deposited material on the surfaces of damaged optical components

Chemical constituents of deposited material on damaged optical components surfaces were investigated by Raman spectroscopy. By comparison with the integrated database of Raman spectra, the Raman spectra of these deposits can be matched with Raman spectrum of carbon, which is a basic chemical element contained in organic compounds. This suggests that the deposited material on surfaces of damaged optical components may be organics. Figure 6(a) and 6(b) show the Raman spectra of deposits on the QWP surface and WG surface of the PC in the Regen, respectively.

It can be concluded from the three experiments illustrated above, that the origin of damages in the Regen is LIC rather than LID, since damage thresholds of optical components used in Regens is much larger than the peak energy fluence in the Regen cavity, and deposited material which contains element carbon has been discovered on the damage sites of optical components surfaces.

3.1 Measurement of concentrations of airborne molecular compounds in the Regen cavity

It has been reported that interaction between laser radiation and nearby outgassing species results in the LIC of optical surfaces on space. In order to investigate if outgassing of organics is the primary cause of LIC in the Regen cavity, we first measured airborne molecular compound (AMC) concentrations in the regenerative amplifier.

The measurement was done using an XAD-2 tube, connected to an air sampling pump (SKC AIRCHEK 2000). The sampling lasted 60 min at a flow of 2 L/min corresponding to a 120 L air sample in the Regen. After sampling, the sampling tube was closed and kept at a temperature less than $4^{\circ }\text{C}$ until receipt at analytical laboratory, after which they were refrigerated at less than or equal to $4^{\circ }\text{C}$ . Then the sample was extracted and analyzed by gas chromatography–mass spectrometry (GCMS). All sampling and measurement work mentioned above was done by ALS analytical testing CO, LTD.

The total ion current chromatogram (TIC) is shown in Figure 7. The top ten AMCs in the sample, whose concentrations were highest, are listed in Table 2. Concentration of each AMC is less than 10 ppbv, and the total AMC concentration in the sample is a mere 42 ppbv, almost in the same order of magnitude of that in a general air environment, which indicates that the outgassing of organic materials in the Regen is at a very low level.

Figure 7. The TIC for the AMC sampling made in the Regen cavity.

Table 2. The top ten AMCs with highest concentrations.

3.2 Output energy performance of the regenerative amplifier after installation of a clean dry nitrogen purge system

If AMCs in the Regen cavity are generated by outgassing of organics, their concentrations should be decreased when clean dry nitrogen (CDN) is supplied to the inner of a regenerative amplifier, which will make the Regen output energy drop more slowly than it used to in the long-term operation. In the experiment of this section, a CDN purge system is installed in a regenerative amplifier to investigate whether the AMCs stem from outgassing of organic materials.

The layout of the regenerative amplifier with a CDN purge system is shown in Figure 8. Two clean stainless steel pipes are installed near both ends of the Nd:glass laser head, which suffers from LIC frequently. CDN with the cleanliness level less than Class 10 standard is purged into the Regen cavity by these two pipes. The flow rate of CDN purge system is 3 L/min. However, as is shown in Figure 9, when the Regen worked, its output energy declined more than 10% after 26 h of operation, much faster than it used to. Then the energy was compensated to its initial value by QWP2, which had preattenuated output energy of the Regen, but the energy decreasing rate still remained the same.

Figure 8. The regenerative amplifier with a CDN purge system, which is installed in the Regen cavity.

Figure 9. The output energy of the Regen with a CDN purge system working versus time.

Then we turned off the Regen, and uncovered its enclosure. It was found that almost all the optical components with antireflective (AR) coatings were damaged on surfaces. All these damages had been proved to be organic deposits by the optical microscope and Raman spectrometer. It should be mentioned that no deposits have been discovered in reflecting mirrors. And the reason for this may be that the energy fluence on the surfaces of reflecting mirrors is almost twice as much as that on AR coatings, since energy fluence had been enhanced by incident beam and reflected beam, and the high fluence laser beam cleaned the deposits on the surfaces of reflecting mirrors at the beginning of their growth.

It can be deduced from the experiments in Sections 3.1 and 3.2 that outgassing of organic materials is not the primary cause of LIC in the Regen, since: first, the total concentration of AMCs in the Regen cavity is on the same level of that in a general air environment; and second, when a CDN purge system worked in the Regen cavity, the output energy performance of the Regen was even worse than it used to be. The latter implies that there might exist aerosols in the Regen cavity, which flowed in the horizontal direction when the CDN purge system worked.

3.3 Investigating origin of aerosols in the Regen cavity

To investigate how aerosols are generated in the Regen cavity, another regenerative amplifier with no CDN purge system is chosen for experiment. After about a 3-month operation, its output energy has dropped by 50%. When the Regen was turned off and its enclosure was uncovered, some organic deposits were discovered on a surface of QWP2. However, QWP2 was mounted in a vacuum compatible piezo rotation stage with very low outgassing rate. There should be no organic deposits on the surface of QWP2 if LIC stemmed from outgassing of organic materials. Therefore, the appearance of deposits on the surface of QWP2 further validates our inference that outgassing of organic materials is not the primary cause of LIC in the Regen cavity.

It was observed that a fraction of the laser beam (about 0.5%), which propagated from the second end mirror M7 to QWP2, was reflected by the second surface (S2 surface) of QWP2 with AR coatings, as shown in Figure 10(a). The reflective beam, which reflected from S2 surface of QWP2, is called ghost beam. This ghost beam was first transmitted to M7 and reflected by it. And finally, the ghost beam illuminated on the gap of the rotation stage, in which QWP2 was mounted, as shown in Figure 10(b). There was a lot of vacuum compatible grease in the gap. Although the energy of the ghost beam was as low as $50~\unicode[STIX]{x03BC}\text{J}$ , its energy fluence was almost on the same level as energy fluence of the main laser beam ( $1~\text{J}/\text{cm}^{2}$ ), since it was focused twice by the concave end mirror M7. This fluence is high enough to generate aerosols.

Figure 10. (a) The diagram which describes that the ghost beam irradiates on the gap of the rotation stage of QWP2. The solid red line represents propagation of the main laser beam, and the dashed blue line represents propagation of the ghost beam generated by reflection of the main beam from a surface of QWP2. (b) A photo of rotation stage.

Subsequently, it was discovered that in the vicinity of each damaged optical component, ghost beams can be found illuminating on organic materials. For instance, we found that the external aperture stop of the stainless steel window holders (shown in Figure 11) of the damaged PC (Fastpulse Technology, Inc.) was not clean enough. There was a lot of residual lubricant on the aperture stop. The lubricant, which should have been cleaned away before the PC left the factory, might be introduced by the manufacturer when the PC was produced. And unfortunately, the aperture stop was illuminated by ghost beams from PBS3 and QWP1, which might produce aerosols near the PC which led to LIC on WG surface of PC. The same situation also existed in the Nd:glass laser head.

Figure 11. A photo of the PC used in the Regen. The external aperture stop of the stainless steel window holders was marked with a red circle.

Therefore, the generation process of LIC on optical components should be: first, ghost beams irradiate on organic materials near optical components; second, aerosols are generated in air near these optical components by interaction of ghost beams and organic materials; then, the main laser beam illuminates aerosols, which results in photochemical and photothermal reactions of aerosols on surfaces of nearby optical components, and will eventually give rise to damages.