Most multispectral devices adopt multi-aperture and multichannel optical systems to capture the information of the target, thus increasing the size and weight of the whole system. A common-aperture multispectral system makes the system compact. The multispectral imaging system is composed of many optical components consisting of common aperture optics, beam splitters and other optical components for different channels. The beam splitters are one of the critical components of these kinds of multispectral imaging systems. The challenge here is develop such kind of optical coatings which will filter out different wavelength at respective bands. A schematic of multispectral imaging system is shown in Fig. 1. The optical system contains a common aperture and two beam splitters: beam splitter1 (BS1) and beam splitter (BS2). The two configurations of the imaging system are shown in Fig. 1, the detailed design and simulation of these two configurations is discussed in next section.
A. Design of beamsplitter (BS1): One of the critical components of this multispectral optical system is the first beamsplitter (BS1). The output of this beamsplitter will determine the system's overall performance. The greater the transmission and reflection we can achieve, the better our overall output performance and system range. When designing a beamsplitter or filter, we must take into account the appropriate substrate, coating materials, fabrication capabilities of a coating system, and overall fabrication cost of the filter. There are two possibilities in the case of BS1. The first is to design dichroic coating such that MWIR band is in the transmission channel while reflecting the visible and SWIR in the reflection channel. The second alternative is to design the thin-film coating such that the MWIR is in the reflection channel, while the visible and SWIR are in the transmission channel.
For the development of a multispectral imaging system, the two methods presented in Fig. 1 as a viable option. In the Fig. 1a the MWIR channel is illustrated in the transmission band, whereas the visible and SWIR channels are in the reflection band. For the Fig. 1a the suitable substrate materials for BS1 includes MgF2, CaF2, ZnS, ZnSe, and sapphire.
TFCALc3 software was used to design a dichroic4–10 coating with 34 layers of coating materials such as SiO2, TiO2, ZnS, and ThF4 on the front surface of a CaF2 substrate. The front surface of the BS1 has a thickness of 3572nm and the rear surface has a single layer of MgF2 with a thickness of 199nm. In the visible and SWIR channel the average transmission of 83% and 83% is achieved, while in the MWIR channel an average reflection of 89% is obtained. The performance of the BS1 mentioned in Fig. 1a is shown in the Fig. 2.
For the second configuration of an imaging system as illustrated in Fig. 1b, the suitable substrate materials for BS1 includes BK7, MgF2, CaF2, ZnS, ZnSe, sapphire etc. The dichroic coating for the front surface of the beamsplitter (BS1) was designed using (a) all-dielectric and (b)metal-dielectric configurations.
The thin-film coating for the front surface of BS1 (schematic Fig. 1b) was designed using a combination of SiO2, TiO2, ZnS, and ThF4. The thickness of thin film coating for the front surface is 3331nm with 36 layers of SiO2, TiO2, ZnS, and ThF4. The rear surface of the filter was designed using a single layer of MgF2. Average transmissions of 87% is obtained in the visible and SWIR wavelength regions, whereas average reflections of 72% is obtained in the MWIR wavelength region. The transmission curve for BS1 using all-dielectric materials is displayed in Fig. 3.
The combination of metal-dielectric11–12 materials with a non-quarter thickness was used for the design of this beam splitter on BK7 using TFCalc software. The coating materials used in the design includes Al2O3, Ag, Si3N4, and SiO2. The front surface of the filter has a thickness of 608nm while a single layer of MgF2 was used to the filter's rear surface. The thickness of 199nm of MgF2 for the rear surface of BS1 is same for all-dielectric and metal-dielectric coatings. In order to demonstrate the entire spectrum, I have deliberately taken CaF2 as a substrate material in Fig. 4. The same spectrum of Fig. 4 is shown separately in Fig. 5 and Fig. 6. Average transmissions of 86.8% and 84.8%is realized in the visible and SWIR wavelength regions, respectively, while average reflections of 81.28% is attained in the MWIR wavelength region.
B. Design of beamsplitter (BS2): Based on the number of layers, thickness and overall performance of the BS1 we have selected the second option Fig. 1(b) for our imaging system. For BS2 as a result, the transmission channel was selected for the SWIR region and the reflection channel was chosen for the visible region. For the BS2, we have selected BK7 as the substrate material. The beamsplitter was designed for the 45o angle of incidence (AOI) thus splitting the incoming beam into two spectrum bands i.e., reflection (Visible, 400-700nm) & transmission (SWIR, 900-1700nm). The transmission and reflection curve for the BS2 is shown in Fig. 7 and Fig. 8 respectively. The average transmission in the SWIR wavelength region is 90.09%, whereas the average reflection in the visible wavelength region is 88.37%. The coating materials used for the front surface are TiO2 and MgF2. The rear surface of the BS2 beamsplitter was designed with a multilayer of TiO2 and SiO2.