2.1.

Homogeneous Glass Development for IR Optics

In this work, 24 IR-transmitting glasses were developed and labeled NRL-1 through NRL-23 and NRL-103. The physical properties of each are summarized in Table 1. The density of each glass was measured via the Archimedes method with helium gas in a Micromeritics Pycnometer with annealed solid rods $\sim 1\mathrm{cm}$ diameter by 1 cm long. For each glass, the glass transition temperature reported here is the tangent onset temperature of the heat flow curve measured using a TA Instruments Q200 differential scanning calorimeter in hermetically sealed chromated aluminum (Alodine) pans. The coefficients of thermal expansion (CTE) represent the average thermal expansion measured using a pushrod dilatometer.

Table 1

Physical properties of IR-transmitting chalcogenide glasses developed at NRL.

Optical properties are reported in Table 2 and include the IR transmission window, refractive index, thermo-optic coefficient ($dn/dT$), and dispersion for each of the three IR wavebands, short-wave (SWIR, 1 to $3\mu \mathrm{m}$), midwave (MWIR, 3 to $5\mu \mathrm{m}$), and long-wave infrared (LWIR, 8 to $12\mu \mathrm{m}$). The IR transmission window is presented here as the short and long wavelength cut-off edges where the transmission drops to 50% of the maximum. The spectral transmission for each glass was measured using polished flat thin samples (typically 2.0 mm thick) with parallel faces on an Analect FTIR. The refractive index for each glass was measured by ${\mathrm{M}}^3\mathrm{MSI}$ (Escondido, California) using minimum deviation refractometry on both wedge samples (for room temperature measurements) and prism samples (for temperature-dependent measurements between 250 K and 350 K). The refractive index data are summarized here, but complete data are available in the form of a datafile compatible with Zemax OpticStudio and Synopsis CodeV from the authors. Dispersion fit and thermal fit parameters were presented previously⁶ as well as a comprehensive overview of the fitting method. All glasses in this series transmit SWIR, MWIR, and LWIR with the exception of NRL-6, which has only partial SWIR coverage down to about $1.4\mu \mathrm{m}$.

Table 2

Optical properties of IR-transmitting glasses developed at NRL. −3-dB window is defined as the short and long wavelengths, in μm, where the transmission, measured using FTIR on ∼2-mm-thick polished samples, drops to 50% of the maximum. For each waveband, n and dn/dT in ppm/°C are reported for the center wavelength of the band.

Windows have been fabricated from all of the materials above using grinding and polishing, and lenses were demonstrated for several of the glasses using SPDT and PGM, several examples of which are shown in Fig. 1. All examples shown here are aspheric lenses with clear apertures of about 25 mm. The SPDT was performed by OptiCraft, Inc. (Woburn, Massachusetts), and PGM lenses were fabricated by Prof. Allen Yi at Ohio State University (Columbus, Ohio). Optical flats and windows were fabricated by Sydor Optics (Rochester, New York) and J.R. Cumberland, Inc. (Marlow Heights, Maryland). Additional lenses have been fabricated by Rochester Precision Optics (West Henrietta, New York) in diameters up to 50 mm. The glasses in this work have been routinely fabricated in 25- and 55-mm diameter ingots that may be cut into optical blanks. Some glasses have been demonstrated with larger diameters through contracts with commercial glass producers in 100-mm-diameter ingots and 200-mm-diameter plates. While the glasses are compatible with common industrial chalcogenide glass production methods, the materials are not yet commercially available.

Fig. 1

Sample lenses using NRL IR-glasses. (a) Aspheric NRL-6 via SPDT, (b) PGM NRL-12, and (c) PGM NRL-8.

2.2.

PseudoGRIN Lens Demonstration

Many of the glasses developed in this work were designed to have very similar thermophysical properties including thermal expansion coefficient and glass transition temperature ($Tg$) as a surrogate for viscosity behavior. Part of the motivation for that criterion was driven by the desire to thermally bond the glasses to each other to enable bonded achromatic doublets. Such optics are common in systems that operate at visible wavelengths, but are practically nonexistent in the IR. This is due in part to the often extreme mismatch in thermal expansion behavior for common IR materials and a lack of commercially available IR transparent adhesive in spite of historic^29–31 and more recent^32,33 work in this area.

In an attempt to demonstrate thermal bonding with these materials, a two-element imaging lens having a 22-mm focal length, $f/\#=1.5$, and a clear aperture of 20 mm was designed where the front optic comprises four layers of NRL glass bonded together without adhesive.³⁴ The optical design, shown in Fig. 2(a), is termed “pseudoGRIN” as it contains a discontinuous refractive index distribution. This unique front element was fabricated by thermally bonding disks of NRL-10, NRL-8, NRL-2, and NRL-6 to each other without any adhesives to form a cylindrical bonded blank. The blank was then molded on a prototype lens molding machine using a concave/convex die set with spherical curvature in order to impart curvature to the three internal conic optical surfaces. Of the 10 blanks that were molded, 4 were sectioned using a low-speed diamond saw and the internal interfaces were mapped using optical microscopy. In order to account for minor deviations from the intended optical design, the measured curvature of the internal interfaces was fed back to the optical design, and the external surfaces of both the multilayered front element and the homogeneous rear element were reoptimized. The aspheric front and rear optical surfaces were formed by SPDT, as were the aspheres on the second, NRL-6, element. The lens was paired with an LWIR thermal imaging camera, FLIR systems Tau2, for demonstration purposes. Test subjects included an adult man and a United States Air Force (USAF) 1951 3-bar template placed in front of an extended source blackbody [Figs. 2(b) and 2(c), respectively]. This is the first successful demonstration of IR imaging using a lens of this type, and it should be noted that none of the lens elements were outfitted with antireflective (AR) coatings.

Fig. 2

(a) Ray tracing schematic of multilayered imager lens design where the front lens element comprises four different IR optical glasses. Sample images from (b) the lens demonstrator include an adult man and (c) a USAF bar chart mask in front of a blackbody source.

Although the “pseudoGRIN” lens design demonstrated here is of limited practical utility, its demonstration serves to validate the homogeneous IR glasses for use in more conventional optics and paves the way for development of true diffusion-based GRIN using these materials. For example, the pseudoGRIN lens validates the capability to thermally bond these new IR glasses without the need for specialty adhesives. The bonded glass body is sufficiently durable to be successfully comolded using conventional PGM tooling. In addition, bonded and molded glasses can be resurfaced using conventional SPDT machining without delamination or fracture, even with internal layers about 1.0-mm thick.

Further validation was realized recently by an engineering team at Rochester Precision Optics (RPO) who, in 2018, designed³⁵ and subsequently demonstrated in 2019^29,30 a dual-band MWIR–LWIR $3\times$ zoom lens using homogeneous IR glass materials supplied by NRL. Their design criteria were a $3\times$ continuous optical zoom lens with a 50- to 150-mm focal length range and $f/3$ operating from 3.5 to $11.5\mu \mathrm{m}$. Jamie Ramsey et al. showed that a lens made entirely from commercially available optical materials (IRG-22, 23, 24, 27, germanium, GaAs, and ZnS) would require 21 lens elements and have a total optical mass of 416 g for the “baseline design.” By incorporating the new NRL glasses into the “SWaP design” in both singlet and bonded doublet forms, the lens element count was reduced to 12 and the total optical mass was halved. The materials used were IRG-24, 27, RPO classic 2, NRL-4, 6, and 8, and ZnS. Notably, germanium and GaAs were eliminated from the system leaving ZnS as the only crystal. Since the lens element count was drastically reduced, the number of surfaces requiring AR coating was cut in half, potentially reducing costs and increasing optical transmission.

3.1.

Diffusion-Based Process

The IR-GRIN fabrication technique developed at NRL is based on diffusion of chalcogenide glasses¹⁸ and is schematically shown in Fig. 4. First, separate homogeneous glass elements with different elemental compositions are combined in a prescribed orientation in a cleanroom environment. The elements, in this case thin glass plates, are then bonded together at an elevated temperature without any adhesives or voids at the interfaces to produce a preform with discrete layers. The preform is then heated to a prescribed temperature and time schedule to allow the homogeneous glasses to interdiffuse across their interfaces. The resulting GRIN blank has a continuously graded composition and refractive index profile in the direction of the optical axis. This axial GRIN blank can then be molded into a hybrid-spherical GRIN blank using compression molding or slumping between hemispherical molds, which imparts curvature not only to the outer surfaces but also to the internal iso-indicial planes. A hybrid or spherical GRIN profile has components of both axial and radial GRIN, and therefore, can provide optical power. SPDT is then used to generate refractive external surfaces on the axial or hybrid-spherical IR-GRIN blank to form a lens element.

Fig. 4

(a) The process for fabrication of IR-GRIN optical elements starts with the fabrication of a layered preform comprising several homogeneous glass elements with different elemental compositions. (b) The preform is heated to a prescribed temperature for a prescribed time to allow the glass elements to interdiffuse forming a GRIN blank with a continuously graded internal refractive index profile. (c) Molding is used to deform the axial GRIN blank into a spherical one. (d) The spherical GRIN blank can then be made into a GRIN lens through conventional means.

The IR-GRIN process employs homogeneous chalcogenide glasses as their optical and physical properties can be accurately controlled by tailoring their compositions, and the glasses used are a subset of those developed in Sec. 2. Specifically, the homogeneous multicomponent chalcogenide glasses used in the fabrication of IR-GRIN and NRL-11 through NRL-22, are a continuous blend of the NRL-1 and NRL-23 endpoint glasses, making the full $\mathrm{\Delta }n$ available for use in IR-GRIN profiles $\sim 0.4$ as shown in the dispersion curves in Fig. 5. The glasses are well-behaved in that there exists a smooth transition in the optical and physical properties through the compositional space. In other words, the glass properties in general (viscosity, glass transition temperature, IR transmission window, density refractive index, and dispersion) vary smoothly and predictably with composition and can be evaluated directly from the compositional gradient as presented elsewhere.^36,37

3.2.

Diffusion Modeling

Diffusion behavior in the chalcogenide glasses used here follows Fick’s laws of diffusion

Diffusion couplets were prepared by first polishing and then thermally bonding pairs of 2.0-mm-thick glass plates comprising each of the endpoint glasses, NRL-1 and NRL-23. The couplets were then heated in a controlled environment for a series of times and different temperatures. The resulting diffused couplets were sectioned and polished, and the compositional gradient was analyzed using electron probe microanalysis (EPMA). The concentration profiles of the individual atomic constituents were mapped using wavelength-dispersive spectroscopy linescan with steps between 1 and $15\mu \mathrm{m}$ depending on the amount of diffusion observed, an example of which is shown in Fig. 6. The EPMA linescan measurement is a destructive technique and was not performed on these specific samples prior to the diffusion step. During process development, EPMA was used on undiffused samples to confirm that the thermal bonding step, which takes place at a lower temperature and does not introduce any diffusion within the spatial capabilities of the instrument ($\sim 1\mu \mathrm{m}$). Since the multicomponent glasses in the IR-GRIN process were carefully designed to be a blend of the two endpoint glass compositions, only the relative concentration of either NRL-1 or NRL-23 glass, rather than the concentrations of the individual elements, needs be considered. In the example, the position of the original interface is indicated by the dashed vertical line at a $\text{distance}=0\mu \mathrm{m}$ and the red and green dots indicate the relative concentration of NRL-1 glass and NRL-23 glass, respectively, after diffusion. The diffused region of the couplet in this example is almost $1400\text{-}\mu \mathrm{m}$ wide, and the asymmetry in the curves is evidence that diffusion is faster where the concentration of NRL-23 is higher and that the diffusivity is concentration dependent. Although the data presented in Fig. 6 were obtained using the NRL-1 and NRL-23 endpoint glasses, the diffusion process creates a continuous concentration gradient, which, in effect, creates regions within the sample having compositions identical to the glasses NRL-11 through NRL-22 as well as everything in between. These diffusion couplets thus capture the diffusion information for all of the glass compositions. The concentration-dependent diffusivity, $D_0(\varphi )$, was calculated from the diffusion profile using Eq. (2) and the Boltzmann–Matano method¹³ and is shown in Fig. 7. In this glass system, the diffusivity was found to vary by a factor of $>2$ within the compositional range of the system.

Fig. 5

Refractive index and dispersion were measured for the homogeneous glasses developed for IR-GRIN and include NRL-1, and NRL-11 through NRL-23. The maximum $\mathrm{\Delta }n$ available is 0.4.

Fig. 6

EPMA was used to profile the interdiffusion of NRL-1 (red dots) and NRL-23 (green dots) glass plates heated for five days in this example. The dashed line at distance = 0 indicates the position of the interface between the two glasses prior to diffusion.

Fig. 7

Concentration-dependent diffusion coefficient, $D_0(\varphi )$, in the NRL-1/NRL-23 GRIN glass system.

In order to simulate axial GRIN diffusion in an arbitrary layered preform, a one-dimensional finite volume method was employed. The preform is divided into many small segments in the direction of the gradient with lengths on the order of $<10\mu \mathrm{m}$. The concentration-dependent diffusivity, $D_0(\varphi )$, is used to calculate the flux of NRL-1 and NRL-23 at the boundary of each segment according to Fick’s first law [Eq. (1)] as well as the new concentration of each segment, as shown schematically in Fig. 8. This is then repeated for many small time increments, $\mathrm{\Delta }t$, such that the corresponding diffusion distance, $2{(Dt)}^{1/2}$, is less than the length of the segment, $\mathrm{\Delta }x$, satisfying the Courant–Friedrichs–Lewy condition.³⁸ The simulation was validated against the diffusion couplets, such as the one shown in Fig. 6, and other partially diffused stacks of glass plates including intermediate compositions (e.g., NRL-11, 12, and 13).

Fig. 8

Schematic for simulating diffusion in one-dimension using a finite volume technique.

This simulation forms the basis for a prescriptive diffusion model. This model was developed to prescribe how a GRIN preform stack should be constructed in order to achieve a desired profile in an axial GRIN preform upon diffusion. To accomplish this, an optimization routine was employed that iterates the diffusion simulation varying both the thicknesses and concentrations of the internal layers and calculates the net deviation from a target concentration profile for each iteration. It is important to note that the diffusion simulation and optimization operate in the realm of concentration and not refractive index. Therefore, the GRIN profile designer must consider the dispersion of the material when specifying the target GRIN profile and selecting the endpoint glasses from within the glass series. The optimizer generates a first-guess preform stack by dividing the target concentration profile, a map of $\varphi (x)$, into $\sim 1\text{-}\mathrm{mm}$-thick plates and averaging the concentration over each plate. Diffusion is then simulated over the entire stack and a deviation profile, the difference between the diffusion profile and the target profile, is calculated. The magnitude of the deviation profile is used as a scoring metric to optimize the stack. It is intuitive that the concentration profile can be made smooth by diffusing for a very long time. However, this tends to homogenize the profile. An optimum diffusion time for the stack is determined when the magnitude of the deviation profile starts to increase as fidelity is lost. The optimization routine iterates this process optimizing both the thickness and composition of the glass plates until the deviations are below the specified target.

Deviations of $\pm 0.001$ refractive index units are typical targets, but this can be reduced by increasing the number of layers and reducing their thicknesses. The prescriptive diffusion model has been adapted to radial profiles but validation is reserved for future work. Hybrid GRIN blanks with spherical internal curvature are not considered in the diffusion model as they are generated by slumping or molding an axial GRIN blank after the diffusion step.

5.1.

Single-Band Compact LWIR

The single-band LWIR design form represents a compact, low-cost imaging application with a 14-mm focal length and $f/0.85$ lens imaging over 8.5 to $11\mu \mathrm{m}$. The overall length is constrained to under 15 mm. Three designs are shown in Fig. 16. The baseline design (a) consists of two IRG-26 glass lens elements with aspheric surfaces, fabricated by PGM, and low-cost was prioritized at the expense of degraded performance (d). An improved design (b) with the same configuration achieves diffraction-limited performance (e) through the addition of a diffractive optical element surface on the rear of the front element. In the GRIN design (c), the rear element is replaced with a hybrid-spherical IR-GRIN element ($\mathrm{\Delta }n=0.17$), and the diffractive surface was eliminated from the front element. In this use case, the incorporation of an IR-GRIN element enables diffraction-limited performance (g) without the transmission penalty associated with the diffractive surface and reduced element weight by about 10%.

Fig. 16

Compact LWIR imager designs (a) baseline, (b) diffractive design, and (c) GRIN design and (e)–(g) their respective MTFs.

5.2.

Simultaneous Dual-Band MWIR/LWIR Cryogenically Cooled FPA

A notional dual-band application utilizing a cryogenically cooled FPA with $1280\times 720\text{pixel}$ format, $12\mu \mathrm{m}$ pixels, 17.6-mm diagonal, simultaneous imaging in the MWIR and LWIR wavebands, 3.5 to $5.0\mu \mathrm{m}$, and 7.8 to $10.5\mu \mathrm{m}$, respectively, is considered. The optical train also incorporates a cold shield as an aperture stop inside the Dewar. The design baseline is a fixed 50-mm focal length, $f/4$ lens, and the $f/\#$ was decreased incrementally to $f/1.5$ in an attempt to assess the impact of these new materials at lower $f/\#$ where the design is less forgiving.

The baseline $f/4.0$ lens was designed using only conventional materials and contains three crystalline elements (from front to rear): ZnSe, germanium, and ${\mathrm{BaF}}_2$, and its MTF was nearly diffraction-limited in both the LWIR and MWIR. Redesigning this lens with the new materials, specifically a doublet front element comprising NRL-8 and NRL-6 and a ZnS rear element, yielded a diffraction-limited lens with two fewer surfaces requiring AR coating. There are potential cost savings to be realized by eliminating germanium in favor of glass and reducing the AR-coating operations. In this design, the rear element could be eliminated, leaving only the NRL-8/NRL-6 bonded doublet if the system requirement can tolerate a slightly degraded MTF in the MWIR for the benefit of a much simpler lens design with about 50% weight savings and only two surfaces to be AR-coated for further reductions in SWaP-C.

At $f/3.0$, the crystal design required four elements (from front to rear): ZnSe, germanium, ${\mathrm{BaF}}_2$, and germanium to achieve a diffraction-limited MTF. An equivalent performing lens was achieved using a bonded-doublet (NRL-8/NRL-6) front element and a ZnS rear element. A true two-element solution was found with NRL-4 front element and an IR-GRIN ($\mathrm{\Delta }n=0.04$) rear element, but the MTF was degraded in the MWIR. As the aperture of the design is increased (lower $f/\#$), the benefits of the new materials and especially IR-GRIN become more significant. At $f/1.5$, the MTF performance of the four-element baseline design, Fig. 17(a), is degraded the LWIR, Fig. 17(b), and MWIR wavebands, Fig. 17(c). Utilizing the new materials, a 50-mm, $f/1.5$ lens with three physical elements (from front to rear): NRL-8/NRL-6 bonded doublet, a ZnS singlet, and an IR-GRIN rear element with a $\mathrm{\Delta }n=0.12$, was designed. The lens design is shown in Fig. 18(a). The radial GRIN profile for the IR-GRIN element is shown in Fig. 18(b) where each color represents the GRIN profile at different wavelengths as indicated in the legend. In this doublet + singlet + GRIN design, the MTF performance was recovered in the LWIR and MWIR wavebands as shown in Figs. 18(c) and 18(d), respectively, and is nearly diffraction limited in both wavebands. The doublet + singlet + GRIN lens is 18% shorter, 15% lighter in weight, and has two fewer surfaces for AR coating. A third lens was designed exclusively using IR-GRIN elements as shown in Fig. 19(a). This all-GRIN design employed front and rear IR-GRIN elements with different complementary radial GRIN profiles as shown in Figs. 19(b) and 19(c), respectively. The GRIN profiles are complementary in that the front element ($\mathrm{\Delta }n=0.06$) has its low refractive index at the center and the rear element ($\mathrm{\Delta }n=0.25$) has its high refractive index at the center. The all-GRIN lens is similar in size and weight to the doublet + singlet +GRIN design. The MTF performance of the all-GRIN lens design exceeds that of the baseline design, in both the LWIR and MWIR wavebands as shown in Figs. 19(d) and 19(e), respectively, but is slightly degraded when compared to the doublet + singlet + GRIN design of Fig. 18.

Fig. 17

(a) Baseline dual-band MWIR-LWIR 50-mm, $f/1.5$ lens design having four conventional elements, from front to rear, ZnSe, germanium, ${\mathrm{BaF}}_2$, and germanium. The MTF of this design was calculated for (b) the LWIR and (c) MWIR wavebands.

Fig. 18

(a) Redesigned 50-mm, $f/1.5$ lens design utilizing the new NRL materials, from front to rear, bonded NRL-8/NRL-6 doublet, ZnS, and IR-GRIN with a radial profile (b). The calculated MTFs in (c) the LWIR and (d) MWIR are improved over the baseline design and approach the diffraction limit (dotted black line).

Fig. 19

(a) An advanced 50-mm, $f/1.5$ lens design utilizes only two lens elements: (b) a front IR-GRIN with a radial profile and (c) a rear IR-GRIN with a complementary radial profile. (d) The calculated MTF is nearly diffraction limited in the LWIR waveband and has good performance in (e) the MWIR waveband for most of the field, excluding the periphery.

This design exercise highlights some of the advantages and limitations of designing with these new IR glasses in GRIN and doublet form. While the all-GRIN design contains fewer lens elements, which reduces the number of surfaces needing AR-coatings and allows for a much-simplified assembly, the MTF performance, while better than the baseline design, is slightly degraded compared to the doublet + singlet + GRIN design. The use of the NRL glasses enables bonded doublets and IR-GRIN to be used alongside conventional materials for smaller, lighter optics with better performance and fewer surfaces requiring AR-coatings, especially at low $f/\#$.

5.3.

Filter-Separable Dual-Band MWIR–LWIR Uncooled FPA

Another application utilizing a notional dual-band uncooled FPA was considered. Arrays of this type are broadband (3.5 to $12\mu \mathrm{m}$), and the MWIR and LWIR bands are only separable using filters. The notional FPA considered here is $640\times 480\text{pixels}$ with a $12\text{-}\mu \mathrm{m}$ pixel size and a 10-mm diagonal. As the FPA is uncooled and lacks a cold shield, the aperture stop may be located anywhere in the lens. As the small pixel size necessitates a fast $f/\text{number}$ lens, $f/1.4$-$f/1.0$ were investigated for the 50-mm focal length lens.

Since the FPA in this example is capable of LWIR capture out to $12\mu \mathrm{m}$, materials such as ${\mathrm{CaF}}_2$ and ${\mathrm{BaF}}_2$ were eliminated from use. The baseline 50-mm, $f/1.4$ lens contained four crystalline elements (from front to rear): ZnSe, GaAs, ZnS, and GaAs as shown in Fig. 20(a). The combined MWIR–LWIR MTF was calculated for the baseline design, shown in Fig. 20(b), and indicates a poor performing lens, although the performance is consistent across the field. In this design exercise, lens designs utilizing the NRL materials were constrained to two physical lens elements and an overall length shorter than that of the baseline lens in an attempt to provide significant size and weight reductions. A 50-mm, $f/1.4$ lens utilizing the NRL materials was designed having an NRL-8/NRL-6 doublet front element and a ZnS rear element and is shown in Fig. 21(a). This lens was much more compact, 27% shorter in overall length, and 40% lighter in weight compared to the baseline lens and had much better performance as the combined MTF, Fig. 21(b), was approximately diffraction-limited across the field. Figure 22(a) shows the design of an all-GRIN lens with two different front and rear IR-GRIN elements. The radial GRIN profiles of the two elements in this design, (c) and (d), respectively, are complementary in that the front element ($\mathrm{\Delta }n=0.06$) has its low index at the center and the rear elements ($\mathrm{\Delta }n=0.09$) have their high index at the center. This all-GRIN lens is 13% shorter in overall length compared to the baseline. The combined MTF for the all-GRIN $f/1.4$ lens is shown in Fig. 22(b) and was significantly better than the baseline, but slightly degraded compared to the doublet + singlet lens, notably at the periphery of the field.

Fig. 20

(a) Baseline, all-crystalline, 50-mm $f/1.4$ lens designed for a notional uncooled dual-band MWIR/LWIR detector with four elements: ZnSe, GaAs, ZnS, and GaAs (front to rear). (b) The calculated MTF for this lens at the full 3.5- to $12\text{-}\mu \mathrm{m}$ combined spectrum.

Fig. 21

(a) A 50-mm $f/1.4$ lens using the NRL materials in a doublet + singlet configuration has a bonded NRL-8/NRL-6 doublet and a ZnS rear element (front to rear). (b) The combined MTF for this design is nearly diffraction limited across the full 3.5 to $12\mu \mathrm{m}$ spectrum.

Fig. 22

(a) A 50-mm $f/1.4$ lens in an all-GRIN configuration utilizes a front GRIN with $\mathrm{\Delta }n=0.06$ and a complementary rear GRIN with $\mathrm{\Delta }n=0.09$. (b) The combined MTF is nearly diffraction limited in the center of the field, with some distortion at the periphery. The radial GRIN profiles of (c) the front and (d) rear elements are complementary.

The doublet + singlet and all-GRIN lens design forms were used as starting points that were reoptimized for wider aperture lenses at $f/1.2$ and $f/1.0$. The baseline all-crystalline lens, due to its extremely poor performance at $f/1.4$, was not developed further at the more challenging lower $f/\text{numbers}$. At $f/1.2$, the doublet + singlet MTF performance, shown in Fig. 23(a), was similar to the $f/1.4$ version, Fig. 21(b), but slightly degraded. An attempt was made to restore performance by replacing the ZnS singlet with an IR-GRIN element ($\mathrm{\Delta }n=0.06$) to make a doublet + GRIN design. The combined MTF, Fig. 23(b), did not change very much; it was slightly improved at the center of the field and some astigmatism was introduced. The $f/1.2$ all-GRIN design, with a front element $\mathrm{\Delta }n=0.06$ and a rear element $\mathrm{\Delta }n=0.14$, had a combined MTF, Fig. 23(c), similar to its $f/1.4$ counterpart, Fig. 22, but with some astigmatism and degradation at the periphery of the field. Opening the aperture to $f/1.0$ begins to reveal the limitations of these design forms as shown in Figs. 23(d)–23(f), largely due to the low lens element count design constraint. The doublet + singlet lens design suffered the largest degradation in its combined MTF, Fig. 23(d). The MTF of the $f/1.0$ doublet + GRIN ($\mathrm{\Delta }n=0.04$) design, shown in Fig. 23(e), is similar to the $f/1.0$ doublet + singlet design, but with reduced astigmatism and blur at the periphery. The MTF of the $f/1.0$ all-GRIN lens design as shown in Fig. 23(f) is only slightly degraded compared to its $f/1.2$ counterpart, most likely due to the more severe index gradient of the rear element ($\mathrm{\Delta }n=0.25$), but is only a slight improvement over the $f/1.0$ doublet + GRIN design. It should be noted that even though the combined MTF of these designs at $f/1.0$ is not quite diffraction-limited, all designs outperform the baseline design at $f/1.4$. This implies a significant potential for improved lens performance in a reduced footprint compared to conventional materials in more demanding applications.

Fig. 23

Combined MTFs calculated for 50-mm dual-band lenses designed for an uncooled 3.5 to $12\mu \mathrm{m}$ detector in six different configurations: (a) $f/1.2$ doublet + singlet, (b) $f/1.2$ doublet + GRIN, (c) $f/1.2$ all-GRIN, (d) $f/1.0$ doublet + singlet, (e) $f/1.0$ doublet + GRIN, and (f) $f/1.0$ all-GRIN.

In an attempt to recover the performance lost at $f/1.0$, the lens element count constraint was relaxed and an additional singlet was added to the doublet + singlet design form of Fig. 23(d). The resulting doublet + 2 singlet design is shown in Fig. 24(a) and contains (from front to rear): an NRL-8/NRL-6 bonded doublet, a ZnS singlet, and an NRL-6 singlet. The combined MTF of this lens is shown in Fig. 24(b) and approaches the diffraction limit.

Fig. 24

A 50-mm $f/1.0$ dual-band lens in a doublet + 2 singlets configuration (a) based on the doublet + singlet design shown in Fig. 21(a) and has three physical elements: a bonded NRL-8/NRL-6 doublet, a ZnS singlet, and an NRL-6 singlet (from front to rear). (b) The combined MTF of this lens is approximately diffraction limited.

This design exercise shows that for dual-band applications requiring low $f/\text{numbers}$, the incorporation of these new materials alongside conventional materials can result in compact lenses with combined MTF performance near the diffraction limit. Increasing the $\mathrm{\Delta }n$ of GRIN elements can be used to improve or maintain performance at low-$f/\#$ but can introduce some astigmatism. One unexpected result was that the doublet + 2 singlet design of Fig. 24 at $f/1.0$ significantly outperformed the baseline all-crystal design $f/1.4$ lens with an equivalent number of refractive surfaces. Using these materials allows for more compact, lower-$f/\#$ dual-band optics than are possible with conventional materials. In addition, GRIN and bonded doublets allow for simpler lens assemblies since the optical complexity has been offloaded to the lens element construction.

5.4.

Broadband XSWIR/MWIR/LWIR

Broadband lens designs, for example, those tasked with operation over SWIR, MWIR, and LWIR wavebands, can be become very complex with many elements.¹ As the target wavelength range becomes large, chromatic aberration becomes the limiting obstacle. This is compounded further by the small number of broadband compatible refractive materials. In this challenging design case, a notional $640\times 512$ uncooled FPA with $12\text{-}\mu \mathrm{m}$ pixels and a 10-mm diagonal, operating at 1.5 to $12\mu \mathrm{m}$ was considered. The lens design requirement was a 50-mm focal length, $f/1.2$. A baseline design using only conventional refractive materials was not achieved. The lens design using NRL materials, shown in Fig. 25(a), contains three physical elements (from front to rear): an NRL-8/NRL-6 bonded doublet, an IR-GRIN element, and a ZnS rear element. The IR-GRIN element has a mild radial profile, Fig. 25(b), with $\mathrm{\Delta }n=0.03$. In this design concept, the surfaces were optimized for separable FPAs resulting in an MTF performance that is approximately diffraction limited in LWIR and extended short-wavelength infrared (XSWIR) wavebands, Figs. 26(a) and 26(c), respectively, and slightly degraded in the MWIR waveband as in Fig. 26(b). This design form was also optimized for a filter-separable common multiband FPA, which degrades the combined broadband MTF.

Fig. 25

(a) Broadband lens design with a 50-mm focal length, $f/1.2$, and comprises four refractive elements including (from left) bonded NRL-8/NRL-6 doublet, IR-GRIN element, and ZnS rear element. The radial profile of the IR-GRIN element is shown in (b).

Fig. 26

MTFs for the broadband lens optimized for separable FPAs at (a) LWIR, (b) MWIR, and (c) XSWIR wavebands.