A Dichroic Beamsplitter for the Laser Protection of Infrared Detectors

Abstract

The design and fabrication approach of a dichroic beamsplitter to meet the protection requirements of infrared detectors for blinding laser weapons is presented. The dichroic beamsplitter must protect against 1064 and 532 nm lasers and have high transmittance in the detection beam band of 3.6–4.7 µm. In order to realize the protection and antireflection (AR) functions of the dichroic beamsplitter, Ta₂O₅, which has a wide band gap and high thermodynamic stability, was selected as the high-refractive-index material. A multilayer stack was deposited on a silicon substrate by ion-assisted electron beam evaporation. The manufactured dichroic beamsplitter features a high laser-induced damage threshold (LIDT), excellent spectral characteristics in the requested spectral region, and good environmental stability.

1. Introduction

Detectors are the core components of military infrared photoelectric systems used to obtain external information [1,2,3]. However, with the rapid development of modern laser technology, blinding laser weapons have been widely used in military applications. These weapons use a laser beam to interfere with and burn out photoelectric elements, thus causing photoelectric systems such as those used for aiming, observation, and tracking to quickly fail and achieve the purpose of “blinding” [4,5,6]. The mid-infrared (MIR) region is a common window of infrared detectors [7] and is often threatened by blinding laser weapons during the detection process. When aircraft and satellites are investigating the target, they are often flying at a high altitude and receive very weak infrared signals. When coupled with the reflection of the optical elements, the detector receives a poor signal. If the optical element has a high transmittance, the detection efficiency of the detection system will be greatly improved. Therefore, a dichroic beamsplitter (as shown in Figure 1) coated on the infrared detector with both laser protection and MIR-AR functions is particularly important for protecting infrared photoelectric detection systems. Specifically, the dichroic beamsplitter must have three key characteristics: (1) a high LIDT in the laser band to protect the detector [8]; (2) high transmittance in the MIR region to improve the detection efficiency of the detector [9,10]; and (3) excellent environmental stability to ensure that the optical components can withstand the impacts of temperature, sand, acid, alkali, and other harsh environments.

Amotchkina et al. [11] developed near-infrared (770–1050 nm) and mid-infrared (4–8 μm) beamsplitters with high spectral efficiency using ZnS and YbF₃ materials. However, sulfide and fluoride have narrow band gaps and low LIDTs [12,13]; moreover, the YbF₃ film is a typical columnar structure that has many gaps, and the packing density is not high [14], so it is difficult for it to withstand the impact of harsh environments. Guo et al. [15] fabricated a UV-visible-shortwave near-infrared (NIR) cut-off and MIR antireflection coating using Al₂O₃ and SiO₂ materials; however, the Al₂O₃ layers deposited by electron beam evaporation (EBE) are characterized by large absorption in the MIR range, thus causing the transmission efficiency to be low, which seriously affects the detection effect of the detector. At present, there is a lack of relevant research on dichroic beamsplitter that takes into account the excellent damage characteristics of laser bands, high transmission characteristics of MIR, and good environmental stability. For this reason, we investigated the high-performance dichroic beamsplitter for the laser protection of infrared detectors.

In addition, the current research shows that among the materials currently used for laser protection, oxide materials are the preferred materials due to their high melting point, good stability, and high laser-induced damage threshold [16]. Among them, Ta₂O₅ films deposited by ion-assisted electron-beam deposition (IAD) and HfO₂ films deposited by EBE have relatively high LIDTs [17,18,19,20,21]. However, compared with EBE-HfO₂, IAD-Ta₂O₅ has better thermal and chemical stability and smaller absorption in the MIR spectral range and can therefore be widely used in many fields [22,23].

In this study, the main purpose is to design and fabricate a dichroic beamsplitter using multilayer Ta₂O₅/SiO₂ films, which takes into account the damage and high reflection characteristics of 1064 and 532 nm laser bands and the high transmission characteristics of the 3.6–4.7 μm detection beam. It was found that the coating conforms to environmental stability standards and exhibited 99.7% reflectance at 1064 nm, 99.5% reflectance at 532 nm, and over 96% average transmittance in the detection beam region of 3.6–4.7 µm. Additionally, the laser damage threshold test results demonstrate that the LIDTs of the dichroic beamsplitter at 1064 and 532 nm laser bands were, respectively, 36.73 and 14.19 J/cm².

2. Experimental Procedure

A dichroic beamsplitter with high reflectance and a high LIDT in the 1064 and 532 nm laser bands, high transmittance in the MIR region (3.6–4.7 µm) at the angle of incidence (AOI) of 45°, and superior comprehensive performance was designed and fabricated. First, dichroic beamsplitters were designed using Optilayer software (version 15.12, Alexander V. Tikhonravov, Michael K. Trubetskov, and Tatiana V. Amotchkina, Garching, Germany) [24] according to the method proposed in this paper. Then, the designed dichroic beamsplitters were fabricated. The spectral characteristics, damage characteristics, and environmental stability of the dichroic beamsplitters were then characterized.

2.1. Design

By selecting appropriate characterization methods, the nominal refractive indices and extinction coefficients of Ta₂O₅ and SiO₂ thin films in the spectral range of 400–1400 nm, and those of Ta₂O₅, SiO₂, Ge, and Al₂O₃ thin films in the spectral range of 3200–5200 nm, were accurately determined based on the optical characterization of single-layer samples and test multilayers containing a small number of layers [11,25,26]. The silicon substrate was optically characterized in the range of 3200–5200 nm. Figure 2 plots the nominal refractive indices and extinction coefficients of the Ta₂O₅ and SiO₂ layers. Because there was almost no absorption of these two materials in the 400–1400 nm range, the extinction coefficient was negligible. Figure 3 shows the nominal refractive indices of the Ge and Al₂O₃ layers and the silicon substrate.

Based on these data, a multilayer dichroic beamsplitter with high reflectance in the laser bands and high transmittance in the MIR region at AOI = 45° was designed. The initial structure is a periodic structure composed of Ta₂O₅ and SiO₂ as high- and low-refractive-index materials, respectively. This structure can be expressed as {[mH (2-m)L]^N L}, and when m = 0.65, the theoretical spectrum can achieve high reflectance at 1064 and 532 nm at the same time, H and L, respectively, denote the quarter-wave optical thicknesses of the Ta₂O₅ and SiO₂ layers at the central wavelength of λ₀, and N is the period number. During the optimization process, the appropriate design was searched for based on the minimization of a merit function MF, which evaluates the proximity between the actual spectral features and the target spectral features [27]:

$$M{F}^2={\displaystyle \sum }_{j=1}^{32}{\left(\frac{R^{\left(a\right)}\left(\mathit{X};{\lambda }_{1,j}\right)-100\%}{{\Delta }_{1,j}}\right)}^2+{\displaystyle \sum }_{j=1}^{60}{\left(\frac{R^{\left(a\right)}\left(\mathit{X};{\lambda }_{2,j}\right)-100\%}{{\Delta }_{2,j}}\right)}^2+{\displaystyle \sum }_{j=1}^{500}{\left(\frac{T^{\left(a\right)}\left(\mathit{X};{\lambda }_{3,j}\right)-100\%}{{\Delta }_{3,j}}\right)}^2$$

(1)

where $\mathit{X}$ is the vector of layer thicknesses; {${\lambda }_{1,j}$}, {${\lambda }_{2,j}$}, and {${\lambda }_{3,j}$} are evenly distributed wavelength points in the spectral ranges of interest of 516–548 nm, 1034–1094 nm, and 3600–4700 nm, respectively; $\left(a\right)$ denotes the average value of p- and s-polarization at the angle of incidence of 45°; and {${\mathsf{\Delta }}_{1,j}$}, {${\mathsf{\Delta }}_{2,j}$}, and {${\mathsf{\Delta }}_{3,j}$} represents the tolerance between the design value and the target value in the corresponding wavelength ranges. The smaller the tolerance is, the closer the design value is to the target value. Different tolerances (Equation (1)) allow for the strengthening or relaxing of the corresponding target requirements.

The damage characteristics of the dichroic beamsplitter were also considered during the design process. For nanosecond laser pulses, the greater the electric field strength at the air–film interface, the more easily the coating is damaged. The electric field distributions of the dichroic beamsplitter at the wavelengths of 1064 and 532 nm were, respectively, simulated and calculated, as shown in Figure 4. By adjusting the thickness of the matching layer, the interface electric field was reduced, and the LIDT was increased theoretically. By considering all these factors, the theoretical spectrum and thickness profile of the final design are shown in Figure 5. The total thickness of the multilayer system is 7597 nm, which is considered appropriate. If the thickness is too small, the reflectance of the laser band will be affected; if the thickness is too large, the transmittance of MIR will be reduced due to material absorption. It is evident that the structure is relatively regular, and this structure has strong preparation feasibility.

The AR coating was deposited on the back of the substrate, which not only ensured the transmission efficiency of the MIR region but also eliminated the ghost phenomenon of the system. In order to achieve excellent transmittance and less stress, Ge and Al₂O₃ materials, which have a large refractive index contrast and small absorption, were selected. The equivalence concept associated with symmetrical multilayers, including Al₂O₃/Ge/Al₂O₃, can be used to obtain a multilayer stack as an original design [28]. Figure 6 presents the optimal results of alternating high- and low-refractive-index layers of different thicknesses after optimization.

2.2. Fabrication

An Optorun electron-beam deposition plant was used for the fabrication of the dichroic beamsplitters, which were deposited on monocrystalline silicon substrates (Φ25.4 × 3.2 mm) by IAD at 250 °C with a base pressure of 2.4 × 10⁻⁴ Pa. During the IAD deposition process, the layers were densified with a 23 cm RF-type ion source, and the parameters of the ion source were as follows: beam current 1000 mA, beam voltage 900 V, and accelerator voltage 600 V. An indirect monochromatic back-reflection optical monitor was used to control the optical thickness of the deposited layers. In order to reduce the accumulation of thickness errors, several monitoring chips were used instead of one monitoring chip. Moreover, quartz crystal monitoring was used to feed back the evaporation rate and layer thickness. The deposition rates for the Ta₂O₅ and SiO₂ layers were 0.3 nm/s and 0.8 nm/s, respectively.

The AR coating was fabricated in a Leybold ARES 1110 vacuum coating unit. The Ge and Al₂O₃ layers were deposited by EBE and IAD [29,30], respectively. The substrates were heated to 240 °C in a vacuum chamber for 2 h, and the temperature was kept at 240 °C during the deposition. Before the deposition, the silicon substrate was ion-cleaned for 12 min in the vacuum chamber with the assistance of a Leybold advanced plasma source (APS). The deposition rate and layer thickness were monitored with a quartz crystal monitor. The Ge layer was deposited at the rate of 0.25 nm/s at 6.5 × 10⁻⁴ Pa, and the Al₂O₃ layer was deposited at the rate of 0.3 nm/s at 3.0 × 10⁻² Pa.

2.3. Characterization

After the deposition, the spectral responses of the deposited samples in different regions were measured using an Agilent Cary 7000 spectrophotometer and a Fourier transform infrared spectrometer (BRUKER INVENIO S), respectively. In addition, the LIDTs of the samples were measured with 1064 and 532 nm laser beams output by an Nd:YAG laser. The lasers output at 1064 and 532 nm with pulse durations of 10 and 8.5 ns were worked at 10 Hz, and the spot diameters of the 1064 and 532 nm laser beams at the Gaussian peak of 1/e² were 341.4 and 194.9 µm, respectively. Figure 7 shows the optical path diagram of the laser damage threshold test. The damage tests were performed in the “s-on-1” (s = 10) mode, and the LIDT of the sample was defined by the zero-probability damage energy density. After irradiation, the damaged morphologies of the samples were observed by a Leica-DMRXE microscope. The samples were also subjected to a series of environmental tests.

3. Results and Discussion

3.1. Spectra Measurements

Figure 8 demonstrates the theoretical and experimental optical properties when both sides of the silicon substrate were coated. The reflectance and transmittance represent the average value of the p- and s-polarization. The experimental results indicate that the reflectance at 1064 and 532 nm was, respectively, 99.7% and 99.5%. Moreover, the average transmittance in the range of 3.6–4.7 µm was 96.2%, which would greatly improve the detection efficiency of detecting light in the MIR region. The difference between the theoretical and experimental spectral curves was mainly due to the fitting error of the optical constants of materials and the monitoring error during the deposition process.

3.2. LIDT Measurements and Analysis

Figure 9 records the damage probability of samples irradiated by 1064 and 532 nm lasers with different energies. The LIDT was obtained from damage probability with the s-on-1 method of ISO standard 21254–2 [31,32]. The linear fitting curve is extrapolated to the X-axis, and the intersection point between the curve and the X-axis is zero damage probability. The experimental results show that the damage thresholds at 1064 and 532 nm laser are 36.73 J/cm² and 14.19 J/cm², respectively. The fabricated dichroic beamsplitters can effectively resist the interference and destruction of the corresponding laser-blinding weapon.

Figure 10 shows the typical damage morphologies of the dichroic beamsplitters after laser damage was collected using optical microscopy. It can be observed from the damage topographies that there were black spots in the damage centers of both samples, which represented impurities and defects generated during coating deposition. The generation of film defects is related to factors such as the substrate cleaning and deposition process; coating defects may be introduced if the substrate is not cleaned thoroughly or if it is contaminated during the deposition process. Under laser irradiation, the defects will absorb more incident laser energy, thus causing damage. As the laser wavelength is shortened, the energy of a single photon becomes larger, so the absorption coefficient of defects and impurities under 532 nm irradiation was significantly higher than that under 1064 nm irradiation; thus, the damage threshold at 532 nm was lower than that at 1064 nm. Figure 10a shows that the morphologies of the damage sites generated under 1064 nm laser irradiation were primarily single and had larger sizes, lower numbers, obvious starting points, and strong directional shock waves, and the surface peeling phenomenon was observed. This is because more energy may have intensively accumulated on the individual defects, leading to a sudden increase in the local temperature and stress difference and eventually producing structural damage. In contrast, the damage morphology under 532 nm laser irradiation shown in Figure 10b exhibited a dense number of damage points. The significant absorption effect of local defects leads to a stronger thermal effect and greater local stress difference, which quickly forms smaller craters that further grow, eventually resulting in absorption damage.

3.3. Environmental Test

In addition to the spectral and damage characteristics, the durability and environmental adaptability of the coating are also important indicators. Necessary durability and environmental tests were performed on five different samples as per the specification of MIL-F-48616. The specific contents include the following: (1) Temperature cycle: Two hours each at 55 °C and −40 °C with 30 min change over; (2) Humidity: It was carried out in an environmentally controlled test chamber at 49 ± 1 °C and 95–100% relative humidity for 24 h; (3) Abrasion: A weight of about 1kg wrapped in a cheesecloth was placed vertically on the surface of the coating and rubbed 50 times in the same area; (4) Adhesion: It was carried out by using a scotch tape. A fresh piece of scotch tape was carefully glued on the coating, and then the tape was removed rapidly in the direction normal to the coated surface and repeated several times; (5) Salt spray: Spray salt spray solution with a pH of 6.7–7.4 at 25 °C for 24 h. As can be seen from the sample 1 pictures taken by the camera before and after the experiment shown in Figure 11, the coating does not crack or fall off, which indicates that high-energy IAD can improve the surface mobility and atomic adsorption of molecules and increase the packing density of the coating, so that it is firmly combined with the substrate. The experimental results are summarized in

Table 1. Durability and environmental tests (as per MIL-F-48616).

Test	Specification	Sample No.	Result
Temperature cycle	2 h each at 55 °C and −40 °C with 30 min change over	Sample 1	Passed
Humidity	24 h, 95% to 100% (RH); 49 ± 1 °C	Sample 2	Passed
Abrasion	50 rubs by cheesecloth	Sample 3	Passed
Adhesion	1 pull by scotch tape	Sample 4	Passed
Salt spray	24 h, 6.7–7.4 pH, 25 °C	Sample 5	Passed

.

4. Conclusions

In summary, we addressed several inherent challenges to designing and fabricating a dichroic beamsplitter with a high LIDT, high spectral efficiency, and excellent environmental stability. Suitable materials and an optimal design are particularly important for improving the damage characteristics, spectral characteristics, and environmental stability of the dichroic beamsplitters. Beamsplitters were produced using an indirect monochromatic monitoring strategy and conventional thermal evaporation technology. In subsequent research, the dichroic beamsplitter has also been widely used in various fields, such as medical diagnosis, traffic management, resource exploration, and other aspects of daily life and safety production.