Design of linear variable optical filter for hyperspectral imaging

The design and simulation of a Linear Variable Optical Filter (LVOF) for the visible and near-infrared (VNIR) 450–900 nm region is described in this paper. The LVOF is designed and simulated using an all-dielectric and metal-dielectric-based induced transmission filter. We compared the design approaches based on all-dielectric and metal-dielectric filters in this paper. The LVOF filter based on the metal-dielectric method reduces design and fabrication complexities while also being cost-effective because no band rejection filters are deemed necessary. The metal-dielectric filter reported in the paper also has minimum spectral transmission of 70% as well as an FWHM of 2%–3% of central maxima.


Introduction
Electro-optical (EO) sensors are finding a wide range of applications in civil and military environments, including surveillance, high-performance target search, track, and acquisition, guidance, and satellite imaging for remote sensing [1] applications such as military target detection. Sensors are classified according to the number of spectral bands they have, such as panchromatic (single band), multi-spectral (more than one band), and hyperspectral (more than one band) (a large number of contiguous bands). The spectral discrimination capability of a hyperspectral imaging system is much significantly higher than that of a multispectral imaging system. Hyperspectral imaging is a hybrid of three technologies: remote sensing, spectroscopy, and imaging. The ability to obtain information without making physical contact with the objects to be studied is attributed to it as remote sensing. The study of the spectral component of the electromagnetic spectrum while splitting the light into various small spectral bands is known as spectroscopy. Imaging is accomplished through the use of standard optics in conjunction with appropriate detectors. The hyperspectral imaging system is composed of three basic components: fore optics for radiance collection, a spectral separation device, and a detector. These sensors capture the reflected spectrum of electromagnetic radiation, culminating in a three-dimensional data cube, two in the spatial domain and one in the spectral domain. Long ranges, wide-field coverage, high spatial resolution for better detection, and high spectral resolution for better identification capability are some of the advanced features of the Hyperspectral Imaging Camera. When compared to equivalent spatial resolution broadband devices, hyper-spectral imaging technology has the potential to provide a superior target to background discrimination.
In this paper, a linear variable optical filter for the VNIR (450-900 nm) region has been designed to serve as a spectral separation component for a hyperspectral imaging camera that can be used on earth observation satellites and airborne platforms. Traditional spectrometers based on prisms and gratings will be replaced by LVOF to fulfill both the requirements of small dimensions and low weight [2][3][4]. This paper proposes a spectral separation device [5][6][7][8][9][10][11][12][13][14][15] that has been designed and simulated on a metal-dielectric-based induced transmission filter [16][17][18][19][20][21] and can be fabricated utilizing thin film technology such as sputtering and e-beam evaporation. In this paper, we will discuss various design approaches for realizing this linear variable optical filter.

Linear variable optical filter (LVOF)
To have a lightweight and miniaturized spectral separation device with no moving parts, a graded transmission filter is proposed.
The LVOF is an optical interference filter whose spectral functionality varies along one direction of the filter, compared to a traditional optical filter whose spectral functionality is intended to be identical at any location of the filter. For a narrow band interference filter, there will be a single transmission peak, implying that the coated sample will exhibit the same transmission at all locations. However, in the case of LVOF, the transmission peak will shift from one edge of the sample to the other. The LVOF integrated with a CCD will work as a single chip spectrometer, thus opening up a huge potential for spectroscopic and imaging applications.
With a suitable choice of thin film coating materials, the LVOF can be fabricated for any spectral band. Because the design and fabrication of this device are both complex, there are two basic approaches for fabricating these devices. The first method employs very-large-scale integration (VLSI) technology, while the second employs thin film technology with masking.
The first approach is based on the all-dielectric [22][23][24][25], i.e., the design and fabrication of this filter is based on the concept of the Fabry-Perot etalon. The LVOF consists of a tapered cavity layer sandwiched between two Bragg reflectors or a dielectric mirror. This type of filter is based on the Fabry-Perot etalon concept, but with a tapering in the cavity layer. However, this filter has the disadvantage of having a narrow spectral band [26], and thus can only be used where a limited spectral band is required, i.e., when the thickness of the cavity layer is altered, the transmission peak will not be able to cover the entire spectrum (450-900 nm). The consequence of this is the generation of 3rd order harmonics, which means we get more than one peak at a specified cavity layer thickness, which we don't need. To design and fabricate this filter in the VNIR (450-900 nm) region using the all-dielectric approach, we need to add many band rejection filters or blocking filters at different positions of the LVOF, ramping up fabrication complexities and increasing the cost of a filter because it requires huge infrastructure such as lithography, dry etching, and an e-beam or sputtering system. The first step is to deposit an alternate layer of high and low index material, followed by a cavity layer. Tapering in the cavity layer can be accomplished through lithography, thermal reflow of the photoresist, dry etching of the cavity layer, and subsequent depositing of the mirror layer using alternate layers of high and low index material. If the cavity layer is made of a high index material, the following layer will be made of a low index material. In the VNIR region, the substrate can be borosilicate glass, fused silica, or quartz, while the high and low index materials can be TiO 2 and SiO 2 , respectively. A wide transmission band will necessitate the use of band rejection or blocking filters to suppress the third harmonics generated by the tapered cavity layer. The following section goes over the design and simulation of an all-dielectric-based LVOF.
A metal-dielectric-based induced transmission filter is a second approach to designing an LVOF. The combination of a suitable metallic layer and a dielectric layer will aid in achieving the desired transmission (70%) with a wider rejection band. As a result, we will be able to cover a wider spectral band without using a blocking filter with this induced transmission filter. As a result, the filter's effective cost and overall performance will be preferable to that of an all-dielectric-based LVOF.
The LVOF filter that has been designed and simulated in this paper will be used in an imaging system that includes a fore optics, relay optics, LVOF, and a detector. The fore optics collect the incoming light and focus it on the LVOF with a spot size of ∼4-10 μm. The LVOF based glass substrate has a dimension of 21 mm × 15 mm with a coated area of 15 mm × 9 mm. The linear variation in the filter varies along its width. The minimum transmission required for the filter in the entire VNIR spectrum is greater than 60%. The required filter's spectral resolution in the overall spectrum is 2%-3% of the central wavelength.

Design and simulation of LVOF
The design and simulation of an LVOF using an all-dielectric and metal-dielectric approach would be discussed in this section.

All-dielectric based LVOF
All-dielectric filter is designed using a combination of high and low index materials TiO 2 and SiO 2 . Since these filters have a limited rejection range, therefore, the filter is split into two spectral bands 450-624 nm (LVOF  Filter1), 624-880 nm (LVOF Filter2), and blocking filters, which is at the backside of the substrate as shown in figure 1. The LVOF filter1 design can be seen in table 1, with cavity-layer thickness ranging from 400-715 nm and peak transmission varying from 450-624 nm. The transmission curve of the LVOF filter1 with a cavity-layer thickness of 415 nm shows a peak at 456 nm and a transmission band at 565 nm (figure 2). Since we require a single transmission peak at different positions of a substrate, therefore, we need to add a low pass filter (figure 3) which will remove the peak at 565 nm and transmission band as shown in figure 4. By integrating a lowpass filter1 and a bandpass filter1 (as listed in table 2), we obtain a linear variable optical filter (LVOF filter1) with a peak transmission varying from 450 to 624 nm, as shown in figure 5. The second part of the filter (LVOF filter2 as specified in table 1) has a transmission peak of 624 to 880 nm and two separate bandpass filters2 (as listed in table 3) are deposited on the backside of the substrate, as shown in figure 1. Figure 6 shows the second part of the linear variable optical filter's performance (LVOF filter2). Combining LVOF filter1 and LVOF filter2 on a single substrate provides a linear variable optical filter with transmission peaks ranging from 450 to 880 nm.

Metal-dielectric based LVOF
There are two approaches that can be used to design such a filter. In the first method, a metal layer can be used as a mirror layer, and SiO 2 can be used as a cavity layer, as in all-dielectric-based LVOF. When the thickness of the cavity layer is varied from 105 to 240 nm, transmission peaks at different wavelengths are generated as shown in figure 7. The design [27] of such a filter includes Sub/ 35 Ag/(110-240) SiO 2 /35Ag/220SiO 2 , with the final layer protecting the silver. This metal-dielectric-metal (MDM [28]) filter requires fewer layers than all-dielectricbased LVOF. The metal-dielectric combination offers a broad rejection band, allowing for the eradication of blocking filters. The only disadvantage with using such filters is that the maximum transmission is less than 40% and the FWHM is higher than 5% of the central maxima. As a result, the filter's overall spectral resolution is poor when compared to the induced transmission filter described below. The second approach for designing an LVOF based on a metal-dielectric filter employs an induced transmission filter with metal-dielectric layers. The metal-dielectric is selected in such a way that we can accomplish a wider rejection band while maintaining maximum transmission in the required spectral band. The paper proposes the design of such a filter using a combination of TiO 2 , SiO 2 , and Ag layers. The VNIR band (450-900 nm) filter has a minimum transmission of 70% across the entire spectral region and an FWHM of 2%-3% of the central maxima.
A metal-dielectric-based linear variable optical filter is made up of a 19-layer stack of TiO 2 , SiO 2 , and Ag on a BK7 substrate, as shown in table 4. Figure 8 depicts the refractive index [29] profile of TiO 2 , SiO 2 , and Ag. The above filter was designed using the TFCalc [30] software. The filter is designed at two extreme wavelengths (380 nm and 770 nm), with the thickness of the oxide layer varying linearly and the thickness of the metallic layer remaining constant throughout the filter. A thin mask is placed close to the substrate in the masking mechanism, and the mask can be varied by some motorized mechanism. After achieving the thickness at the lowest design wavelength, in our case 380 nm, the mask can be kept in motion for a specific material until it achieves the final thickness, at 770 nm. After depositing the first material, the mask is returned to its zero position before depositing the second material. The mask will be kept at zero position for the metallic layer, and no mask motion is required because the thickness is constant across the entire filter. The above filter's total thickness required for design and fabrication is 1761 nm, TiO 2 685 nm, SiO 2 980 nm, and Ag 95 nm. As the thickness of the Ag is so thin, it is not very expensive. Silver is a suitable material because it has the highest extinction coefficient when compared to other materials such as gold and aluminum, resulting in wider outband rejection. Figure 9 illustrates the filter's performance at the two extreme ends of the spectral band. Figure 10 describes the LVOF transmission curve for the entire VNIR region. The above-mentioned induced transmission filter has an advantage over the LVOF-based all-dielectric filter in that no blocking filters are required. When compared to all-dielectric filters, which involve significant infrastructures such as lithography, etching, and a physical vapor deposition (PVD) system, the fabrication of such a filter can be done at a very low cost. The fabrication of such an induced transmission filter is simple with the PVD system's masking mechanism, resulting in a lower cost in terms of the number of layers and infrastructure required. The proposed metal-dielectric-based filter has better and more uniform (Transmission 70%) performance across the entire VNIR spectral band than some commercially available filters.

Conclusions
An all-dielectric, metal-dielectric-metal, and metal-dielectric induced transmission filter was being used to model and simulate an LVOF filter covering the VNIR (450-900 nm) spectral band for the hyperspectral imaging camera. The fabrication of an all-dielectric-based LVOF necessitates extensive infrastructure such as lithography, thermal reflow, masking, etching, and PVD. The need for additional blocking filters, as well as a large number of coating layers, raises the cost and complexity of design and fabrication. The MDM-based LVOF filter design requires fewer layers but has a lower transmission and a wider FWHM (>5%). A metal-dielectricbased induced transmission filter serves as the foundation for a straightforward and reasonable approach to the design and fabrication of LVOF. This metal-dielectric-based LVOF has improved spectral resolution and uniform transmission across the entire spectral band, eliminating the need for blocking filters. As a result, a masking mechanism in the PVD system will facilitate the fabrication of LVOF without the use of blocking filters, while covering the entire VNIR spectral band with a transmission of 70% and a spectral resolution of 2%-3% of the central maxima.