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In a compact Nd:Glass resonator, the laser that enters the gain medium at Brewster angle can work either for P-polarization or S-polarization, in which polarization the optical coatings possess higher laser-induced damage threshold (LIDT) was investigated. For the P-polarized configuration, only one high reflection (HR) coating on the rear surface of the Nd:Glass substrate is needed, and the laser-induced damage occurred near the substrate-coating interface at a fluence of 10 ± 2 J/cm2 (1064nm 10ns). Although S-polarized configuration needs two coatings, one HR coating and one anti-reflection (AR) coating on the rear and front surface of the Nd:Glass substrate respectively, its overall LIDT was about 1.8 times higher than that of the P-polarized configuration. The laser-induced damage occurred at the interface between the S-polarized AR coating and the Nd:Glass substrate. The observed interfacial damage behaviors were interpreted using a phenomenological model that took the nano-sized absorbers, electric-field intensity (EFI) distribution and coating thickness into consideration.
© 2016 Optical Society of America
1. Introduction
The resonating cavity is an arrangement of optical coatings that surround the gain medium and it provides feedback of the laser light. For common types of resonating cavities, the cavity mirrors are separated with the gain medium. Whereas, for advanced cavity designs with additional features like compactness, thermal control, etc., the cavity mirrors are deposited directly on the rear surface of gain medium [1–4]. In such configuration, the laser illuminates HR coatings from the substrate-coating interface. Compared to HR coatings irradiated from the air-film interface [5–11], the damage characteristics of this kind of HR coatings are less addressed [12, 13].
Our previous work has investigated damage behaviors of the HR coatings that were irradiated at normal incidence from Nd:YLF crystal-film interface [12]. It was confirmed that nano-sized absorbers in vicinity of the crystal-film interface gave rise to the laser-induced damage. The nano-sized absorbers might come from surface contamination, polishing residues in the redeposition layer, extraction of defects during coating deposition or microstructure mismatch between the film and the crystal [14–17]. The LIDT of a HR coating can be increased either by removing the nano-sized absorbers or by reducing EFI at the crystal-film interface where the nano-sized absorbers are concentrated. Because it is quite challenging to remove all these nano-sized absorbers near the interface, the approach that reduced EFI by changing the coating design was used. An optimal HR coating working at normal incidence was designed to decrease EFI at the crystal-film interface, and it increased the LIDT significantly.
Besides the laser cavity working at normal incidence, there are some kinds of resonating cavities where the laser enters the gain medium or the grating-waveguide structures at oblique incident angles [18–20]. In such resonating cavities, the HR coatings that are irradiated from the substrate-coating interface can work either for P-polarization or S-polarization. It is worth to explore in which polarization the HR coatings possess higher LIDT. Moreover, when an HR coating is directly deposited on the rear surface of the gain medium, an AR coating on the front surface is also needed. Whether the HR coating or the AR coating is more vulnerable to laser-induced damage has never been reported yet. Only after knowing the above two issues, the optimal coating solution for oblique angle incidence in compact resonating cavities can be determined to achieve a higher overall LIDT.
This work explored the optimal HfO2/SiO2 coatings for a compact Nd:Glass resonator where the laser enters the gain medium at Brewster angle. The experimental design, sample preparation and laser damage test are described in section 2. Section 3 presents the comparison and analysis of the damage test results, and then gives a phenomenological model to explain the observed interfacial damage behaviors. Our conclusions are given in section 4.
2. Experiments
2.1 The schematic of a compact Nd:Glass resonator
The configuration of a side-pumped Nd:Glass resonator is given in Fig. 1, where Nd:Glass slab is pumped with 803 nm AlGaAs laser-diode (LD) modules. To have a compact structure, the HR coating was directly deposited on the rear surface of the Nd:Glass substrate. Another merit of this configuration is that the HR coating could be connected to cooling water to achieve better thermal control. In this configuration, the laser enters the gain medium at oblique incidence and it can work either for P-polarization or for S-polarization. It is worth to explore in which polarization the optical coatings possess higher LIDT. More specifically, the Brewster angle incidence was adopted in this work to highlight the difference between the coating solutions for P-polarization and S-polarization. For the P-polarized laser, the front surface is perfectly transmitted and only one HR coating on the rear surface of the substrate is needed. Whereas, for S-polarized laser, it is necessary to have both AR and HR coatings on front and rear surfaces of the Nd:Glass substrate. If the LIDT of the P-polarized HR coating is higher than the overall LIDT of S-polarized optical coatings, it is preferential to adopt the P-polarized configuration because one coating is much easier than two depositions from the aspect of process complexity.
2.2 The coating designs for P-polarization and S-polarization
For both AR and HR coatings that are irradiated from the substrate side, our previous studies have proven that the laser-induced damage was triggered by the joint action from nano-sized absorbers and EFI [12, 21]. It also has been confirmed that the nano-sized absorbers were concentrated in vicinity of the substrate-coating interface. And the LIDT of these coatings could be increased by decreasing EFI at this interface. So in the following discussion, the coating design will be optimized to adjust EFI at the substrate-coating interface. HfO2/SiO2 coatings are used in this study for their good laser damage resistance in the near infrared region.
For the HR coatings that are irradiated from the substrate side, the EFI profile has an oscillating pattern and it varies from peak to valley after each quarter wave optical thickness. It is possible to adjust EFI at the substrate-coating interface without degrading the reflectivity of HR coatings. For the P-polarized configuration as shown in Fig. 2(a1), EFI at the substrate-coating interface is not continuous as given in Fig. 2(a2). It is physically impossible to reduce EFI to zero at this interface. The optimal design is the quarter-wave stack with a half-wave SiO2 layer on the substrate. To achieve a reflectivity higher than 99.5%, a 30 layers design of [Air:(HL)^15L:Sub] was used. The EFI profile has an abrupt jump at the substrate-coating interface with a value of 21% at the coating side and a value of 51% at the Nd:Glass side. Whereas for S-polarization, EFI at the substrate-coating interface is continuous and it can be decreased to zero by changing the coating design, as shown in Fig. 2(b3). The optimal design is also the quarter-wave stack with a half-wave SiO2 layer on the substrate. To exclude the influence of coating thickness on the LIDT, the same 30 layers design of [Air:(HL)^15L:Sub] was used for S-polarization. It is expected that the S-polarized HR coating exhibits higher LIDT than that of the P-polarized HR coating. However, there is one more AR coating on the front surface of the Nd:Glass substrate for S-polarization, as shown in Fig. 2(b1). It is necessary to know whether the S-polarized AR coating has higher LIDT than that of the P-polarized HR coating or not.
Fig. 2 Schematics of (a1) P-polarized configuration and (a2) EFI profile for P-polarized HR coating. Schematics of (b1) S-polarized configuration, (b2) EFI profile for S-polarized AR coating and (b3) EFI profile for S-polarized HR coating.
For S-polarized AR coatings, the EFI profile varies smoothly and gradually. It is physically impossible to reduce EFI at the substrate-coating interface without degrading the transmittance. A two-layers AR design was used. It is worth to note that the laser enters the AR coating from two directions. EFI at the substrate-coating interface is 42% and 45% for air-side and substrate-side incidence respectively. It is quite difficult to estimate whether the S-polarized AR coating or the P-polarized HR coating has a higher LIDT, because their EFI values at the substrate-coating interface are similar. Experimental studies need to be carried out to know in which polarization the optical coatings possess higher LIDT.
2.3 Preparation of the coatings and LIDT testing
The Nd:Glass substrates were first carefully cleaned by ultrasonic cleaning process and HfO2/SiO2 coatings were prepared using the electron beam evaporation process. The details of the cleaning and deposition processes have been given in the previous paper [22, 23]. Photo-thermal technique was used to measure the coating’s absorption at the wavelength of 1064 nm. The absorption of P-polarized HR coating, S-polarized HR coating and AR coating is 6.5 ppm, 5.5 ppm and 6.0 ppm respectively. The LIDT testing was performed using 1.064 μm, 10 ns pulses from a Nd:YAG laser having a TEM00 mode, a beam diameter of 1mm and a repetition rate of 10 Hz. The raster scan method [24] was used to determine the LIDTs of optical coatings. The laser fluence first increased to 24 J/cm2 with a 2 J/cm2 increment, after that it increased to the “damage fluence” with a 4 J/cm2 increment. The damage morphologies may change during the raster scan test, so a single shot method was used to obtain representative damage morphologies with a proper fluence. The damage morphologies were observed and characterized by a Nomarski microscope, a scanning electron microscope (SEM) and a focus ion beam (FIB) equipment.
3. Results and discussion
LIDTs of optical coatings in three configurations are given in Table 1. The overall LIDT of S-polarized coatings is about 1.8 times higher than that of P-polarized HR coating. The laser-induced damage of these optical coatings was carefully examined to reveal the characteristics of the observed interfacial damage. First, the damage behaviors of HR coatings working at P-polarization and S-polarization are compared. The P-polarized HR coating has a LIDT of 12 ± 2 J/cm2 and the S-polarized HR coating has a LIDT of 30 ± 4 J/cm2. This is in accordance with our prediction that the lower EFI at the substrate-coating interface for S-polarized HR coating leads to a higher LIDT. These two HR coatings also have different damage morphologies. Figure 3(a) shows that flat bottom pit is the typical damage morphology for the P-polarized HR coating. The depth of the damage site equals to the coating thickness, and the diameter of the craters is of dozens of micrometers. Obviously, the strong EFI (either 21% or 51%) triggered the damage from the nano-absorbers at substrate-coating interface. The red circles in Fig. 3 are used to show the starting points of the damage. Whereas, the S-polarized HR coating has much deeper and bigger damage site. Figure 3(b) shows that its depth is about tens of micrometers and its diameter is about a hundred micrometers. The damage is associated with the strong EFI in the deep region of the substrate as shown in Fig. 2(b). Since EFI at the substrate-coating interface is close to zero, only the nano-sized absorbers in the deep subsurface region can trigger the laser-induced damage at the depth where EFI is strong. The absorptivity and density of nano-sized absorbers in the deep subsurface region is much less than that in vicinity of substrate-coating interface, so the LIDT of S-polarized HR coating is almost 3 times higher than that of P-polarized HR coating. There is one issue that is worth to note about the LIDT testing of S-polarized HR coating. The reflectance loss of the uncoated front surface of the Nd:Glass substrate is about 15%. So the LIDT of the S-polarized HR coating is corrected with a factor of 85%. It is also worth to note that we observed the redeposited debris from the big damage sites. These debris are more damage resistant than the nano-sized absorbers at the substrate-coating interface or within the subsurface of the substrate. During our raster scan test, we did not observe the damage that was created from the redeposited debris. A preliminary model of interfacial damage between the substrate and the coating can be given: the joint action of EFI and nano-sized absorbers contributes to triggering the laser-induced damage.
Table 1. LIDTs of optical coatings in three configurations
Fig. 3 Typical damage morphologies of (a) P-polarized HR coating irradiated from the substrate-coating interface and (b) S-polarized HR coating irradiated from the substrate-coating interface. The red circles are used to show the starting points of the damage.
As to the AR coating on the front surface of the Nd:Glass substrate for the S-polarized configuration, the damage occurred preferentially at the output region at a fluence of 22 ± 2 J/cm2, as given in the third column in Table 1. It is in accordance with the previous studies that output surface of the AR coating is more vulnerable to laser-induced damage. Although the S-polarized AR coating has a lower LIDT than S-polarized HR coating, the overall LIDT of the S-polarized coatings is still 1.8 times higher than that of the P-polarized HR coating. Their damage characteristics were further investigated to obtain deeper understanding of the interfacial damage.
First, the laser damage resistance of the bare front substrate surface, S-polarized AR and HR coatings is compared. The above preliminary model can perfectly explain the damage testing results of S-polarized AR and HR coatings. S-polarized AR coating has a stronger EFI at the substrate-coating interface where the nano-sized absorbers are concentrated, so it has a lower LIDT. Delaminations are the typical damage morphologies for the S-polarized AR coatings, and the diameter of the damage sites is about a hundred micrometers. Some damage sites have visible starting points, whereas some others have no initiating points, as shown in Fig. 4. For the damage sites without starting points, maybe the initial damage is too small to be observed, or the plasma scald plays a more important role in the delaminations of layers. More importantly, the comparison between the bare front substrate surface and S-polarized AR coating sheds more light on the characteristics of the interfacial damage. The second column in Table 1 reflects that the bare front substrate surface is more damage resistant than S-polarized HR coating. However, the third column in Table 1 shows that the AR coating on the front substrate surface makes substrate-coating interface more vulnerable to laser-induced damage. There are two possible reasons for this. Either new defects are created during the deposition of AR coating or the AR coating plays a detrimental role in the development of laser-induced damage [16, 25, 26]. So the above model of interfacial damage is improved with the comprehension that the coating leads to a degraded LIDT of interfacial damage.
Fig. 4 Damage morphologies of S-polarized AR coating (a) with a visible starting point and (b) without a visible starting point.
The most interesting finding is that the LIDT of the S-polarized AR coating is about 1.8 times higher than that of the P-polarized HR coating. These two coatings are different in two aspects. First, the P-polarized HR coating has discontinuous EFI at the substrate-coating interface with a value of 21% at the coating side and a value of 51% at the substrate side, whereas, EFI of S-polarized AR coating is continuous at the substrate-coating interface with a value of 45%. Discontinuity of EFI at the substrate-coating interface may be a possible reason for the lower LIDT of the P-polarized HR coating. Although it has been reported that EFI discontinuity at the air-coating interface decreased the LIDT of P-polarized AR coating [27], the mechanism of how EFI discontinuity affects the LIDT is still not fully understood. It is worth to do more works to explore this. Second, the P-polarized HR coating has a total thickness of 5.7 um, which is more than 10 times thicker than that of the S-polarized AR coating. Previous studies have proven that the diameter of interfacial damage is proportional to the coating thickness. According to the model of interfacial damage, it is also reasonable to think that the interfacial damage might occur for both S-polarized AR coating and P-polarized HR coating at the same fluence, for example 12 ± 2 J/cm2. However, the damage sites of S-polarized AR coating are tiny and they have negligible influence on the practical performance of the laser coatings. Moreover, such damage sites do not grow under subsequent laser irradiations [28]. We think that the thinner AR coating has a higher practical LIDT than that of the thicker HR coating. So we hypothesize that both EFI discontinuity and coating thickness have a negative effect on the LIDT of the coatings. We also considered whether the nodules in the P-polarized HR coating contributed to the lower LIDT or not. Our raster scan test showed that the damage always started from the nano-sized absorbers at the substrate-coating interface or in the subsurface. We are confident that the nano-sized absorbers at the substrate-coating interface or in the subsurface are more vulnerable to laser damage than nodules. The nodules have no contribution to the two times lower LIDT of P-polarized HR coating.
Now the phenomenological model that takes the nano-sized absorbers, EFI distribution and coating thickness into consideration can explain the observed interfacial damage between the optical coatings and the substrate. It gives some guidance on finding the optimal coatings with higher LIDT, but more works are worth to be done to understand the physical principles behind the proposed phenomenological model.
4. Summary
The interfacial damage between the optical coatings and substrate was investigated to find an optimal coating solution for the compact resonating cavity working at Brewster angle. It was found that strong EFI at the location where the nano-sized absorbers are concentrated was the primary reason for triggering the interfacial damage. EFI discontinuity at the interface may also contribute to decreasing the LIDT. Moreover, the coating plays a detrimental role in the development of interfacial damage. Thicker coating seems to exhibit a lower LIDT. Our proposed phenomenological model indicates that the S-polarized coating solution usually has a higher LIDT than P-polarized coating solution for the compact laser cavity.
Funding
National Natural Science Foundation of China (61522506, 51475335, 61621001, 61235011); National Program on Key Research Project (2016YFA0200900); National Key Scientific Instrument and Equipment Development Project (2014YQ090709); National Major Research Equipment Development project (ZDYZ2013-2).
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