Abstract:
Various embodiments provide an optical system including a first lens group including a plurality of lenses, the first lens group being configured to correct for lateral chromatic aberration; and a second lens group including a plurality of lenses, the second lens group being configured to correct for axial chromatic aberration, the second lens group being disposed adjacent the first lens group. The optical system further includes a detector disposed behind the second lens group; a mechanism for switching a configuration of the optical system between a narrow field of view (NFOV) configuration and a wide field of view (WFOV) configuration; and a ray path steering system disposed in front of the first lens group, the ray path steering system comprising a pair of counter-rotating grisms configured to enhance a field of regard of the optical system. The optical system also includes a stabilization system configured to suppress image jitter, the stabilization system including a mechanism for decentering at least one lens in the first lens group or in the second lens group orthogonal to an optical axis of the optical system. A pupil of the optical system is located external to the first and second lens groups for location of a cold shield within a cryo-vac Dewar enclosing the detector.

Description:
BACKGROUND 
       [0001]    This disclosure pertains to optical imagers in general and in particular to a dual field of view refractive optical system. 
         [0002]    Demand for imaging sensors that provide wide area surveillance is increasing. Wide area surveillance can be used in various applications such as on an unmanned aerial vehicle (UAV) platform for target recognition. Wide area surveillance can be performed at various wavelength ranges depending on the desired application. The wavelength ranges of interest include short wavelength infrared radiation (SWIR) in the wavelength range between approximately 1 μm and 3 μm, mid wavelength infrared radiation (MWIR) in the wavelength range between approximately 3 μm and 5 μm, and long wavelength infrared radiation (LWIR) in the wavelength range between approximately 8 μm and 12 μm. 
         [0003]    In certain applications, it is desirable to provide an optical system with a dual field of view capability: a wide field of view (WFOV) optical system to achieve a large area coverage at coarser spacial resolution and a narrow field of view (NFOV) optical system to achieve a smaller area coverage at finer spacial resolution. However, providing such dual capability may be challenging especially in the case where a small light weight optical package is sought, for example, for portability on UAVs. 
       SUMMARY 
       [0004]    One or more embodiments of the present disclosure provide an optical system including a first lens group including a plurality of lenses, the first lens group being configured to correct for lateral chromatic aberration; and a second lens group including a plurality of lenses, the second lens group being configured to correct for axial chromatic aberration, the second lens group being disposed adjacent the first lens group. The optical system further includes a detector disposed behind the second lens group; a mechanism for switching a configuration of the optical system between a narrow field of view (NFOV) configuration and a wide field of view (WFOV) configuration; and a ray path steering system disposed in front of the first lens group, the ray path steering system comprising a pair of counter-rotating grisms configured to enhance a field of regard of the optical system. The optical system also includes a stabilization system configured to suppress image jitter, the stabilization system including a mechanism for de-centering at least one lens in the first lens group or in the second lens group orthogonal to an optical axis of the optical system. A pupil of the optical system is located external to the first and second lens groups behind the second lens group for location of a cold shield within a cryo-vac Dewar enclosing the detector. 
         [0005]    These and other features and characteristics of the present disclosure, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. In one embodiment of this disclosure, the structural components illustrated herein are drawn to scale. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the inventive concept. As used in the specification and in the claims, the singular form of “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    In the accompanying drawings: 
           [0007]      FIG. 1  depicts an optical imaging system in a wide field of view (WFOV) configuration, according to one embodiment; 
           [0008]      FIG. 2  depicts the optical imaging system in a narrow field of view (NFOV) configuration, according to one embodiment; 
           [0009]      FIG. 3  depicts the optical system with a superposition of NFOV and WFOV optical paths; and 
           [0010]      FIGS. 4 ,  5  and  6  depict two rotating grisms for steering a line of sight into optical system shown in  FIGS. 1 and 2 , according to one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]      FIGS. 1 and 2  depict an optical imaging system providing dual field of view (FOV) in the MWIR range (3 μm to about 5 μm), according to one embodiment.  FIG. 1  depicts the optical system in a wide field of view (WFOV) configuration and  FIG. 2  depicts the optical system in a narrow field of view (NFOV) configuration. As shown in  FIGS. 1 and 2 , optical system  10  comprises first lens group  12  and second lens group  14 . Optical system  10  further includes detector  16  such as, but not limited to, a focal plane array. In one embodiment, the detector  16  is selected to be sensitive in the MWIR wavelength range between about 3 μm and about 5 μm. However, the detector  16  can also be selected to be sensitive in other wavelength ranges such as SWIR and LWIR. Detector  16  is disposed behind second lens group  14 . First lens group  12  and second lens group  14  capture radiation from a far field and focus the radiation onto detector  16 . 
         [0012]    In one embodiment, first lens group  12  comprises first lens element  12 A, second lens element  12 B and third lens element  12 C. In one embodiment, first lens element  12 A has a positive power, second lens element  12 B has a negative power and third lens element  12 C has a negative power. In one embodiment, lens element  12 A, lens element  12 B and lens element  12 C are made from, respectively, Si, Ge, and an amorphous material transmitting infrared radiation (AMTIR), such as AMTIR-1 containing Ge (about 33%), As (about 12%) and Se (about 55%). Although first lens group  12  is described above and depicted in  FIG. 1  as having three lens elements, first lens group  12  can have any number of lens elements, for example, two, three, four or more lens elements. In addition, although the first lens element  12 A, second lens element  12 B and third lens element  12 C are made from the above listed material, these lens elements  12 A,  12 B,  12 C can also be made from other optical material which are selected depending upon the desired range of wavelengths. 
         [0013]    In one embodiment, second lens group  14  comprises first lens element  14 A, second lens element  14 B, third lens element  14 C, fourth lens element  14 D, fifth lens element  14 E and sixth lens element  14 F. In one embodiment, first lens element  14 A has a positive power, second lens element  14 B has a negative power, third lens element  14 C has a positive power, and fourth lens element  14 D has a negative power. In one embodiment, fifth and sixth lens elements  14 E and  14 F have substantially zero power. Fifth lens element  14 E has substantially no curvature and thus substantially zero power. However, fifth lens element  14 E is provided with an aspherical departure to correct for spherical aberration. In one embodiment, sixth lens element  14 F is a plate and is provided as a cryo-vac Dewar window to separate the imaging optics  12  and  14  from the cold shielded detector  16  (provided within a cold shield or Dewar). In one embodiment, lens elements  14 A,  14 B,  14 C,  14 D,  14 E and  14 F are made from, respectively, As 2 S 3 , ZnS, AMTIR1, CaF 2 , Si, and Si. Although third lens group  14  is described above and depicted in  FIGS. 1 and 2  as having six lens elements, third lens group  14  can have any number of lens elements, for example, two, three, or more lens elements. 
         [0014]    In one embodiment, first lens group  12  is provided, inter alia, to correct lateral chromatic aberration. In one embodiment, second lens group  14  is provided, inter alia, to correct axial chromatic aberration. Second lens group  14  is further provided to correct field curvature to achieve a substantially planar focal surface or near zero Petzval sum on detector  16 . For example, by providing a substantially planar focal surface or near zero Petzval sum on the detector this allows to minimize optical aberrations. Lens elements  12 A- 12 C and  14 A- 14 F are centered around axis AA to define the optical axis of optical system  10 . 
         [0015]    In one embodiment, the material from which the various lens elements in the optical system  10  are fabricated can be selected from a material transmitting in the MWIR wavelength range between about 3 μm and about 5 μm. However, the lenses can also be fabricated from materials transmitting in the SWIR, LWIR depending on the desired application. 
         [0016]    As shown in  FIG. 1 , in the WFOV configuration, all the lens elements are traversed by the optical rays to achieve a wide field of view. Whereas, as shown in  FIG. 2 , in the NFOV configuration, lens element  12 C in lens group  12  and lens element  14 A in lens group  14  are not traversed by optical rays to achieve a narrow field of view. In one embodiment, in order to transform optical system  10  from the WFOV configuration into the NFOV configuration, or vice versa, a mechanism is provided to move out the lens elements  12 C and  14 A from the path of radiation or move in lens elements  12 C and  14 A within the path for radiation. Alternatively, lenses  12 A and  14 A can be termed a drop-in Galilean attachment which transforms the NFOV configuration into the WFOV configuration when it is inserted into the optical path. 
         [0017]      FIG. 3  depicts optical system  10  with a superposition of the NFOV and WFOV optical paths, according to an embodiment. Optical system  10  is of a refractive telephoto form with an external rear aperture stop  18  positioned behind lens element or window  14 F. The position of external rear aperture stop  18  corresponds to a position of an external pupil of optical system  10 . In one embodiment, detector  16  is placed in a Dewar (not shown) for cold shielding detector  16 , as generally known in the art. Optical system  10  further comprises flat fold mirror  21 . Fold mirror  21  is used to modify a volume of the optical system from a long cylindrical profile to a shorter but wider profile. The use of flat fold mirror  21  allows to achieve volume reduction for packaging purposes. 
         [0018]    Optical characteristics of optical system  10  are summarized in TABLE 1. In one embodiment, detector  16  is an FPA having 4000 by 4000 pixels. However, detector  16  can also be selected with any number of pixels and with any geometry including, square, rectangular, circular, etc. In one embodiment, a size of one pixel is approximately 10 μm. However, detector  16  can be provided with a different pixel size as desired. In one embodiment detector  16  is selected to operate in the wavelength range between about 3 μm and 5 μm, for example between about 3.8 μm and 4.2 μm. In one embodiment, the FOV achieved by optical system  10  in the WFOV configuration is approximately 26 deg. by 26 deg. (when using a square detector) or a 38 deg. diagonal. In one embodiment, the FOV achieved by optical system  10  in the NFOV configuration is relatively small (e.g., approximately 1 degree or less). In one embodiment, the focal length of optical system  10  in the WFOV configuration is approximately 3 inches and the focal length of optical system  10  in the NFOV configuration is approximately 15 inches. In one embodiment, a speed achieved by optical system  10  in the WFOV or NFOV configurations is approximately F/3. In one embodiment, an aperture of optical system  10  in the WFOV configuration is approximately 1 inch and in the NFOV configuration approximately 5 inches. In one embodiment, an instantaneous field of view (IFOV) of optical system  10  in the WFOV configuration is approximately 130 μrad and the IFOV of optical system is approximately 26 μrad. 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 PARAMETER 
                 WFOV 
                 NFOV 
               
               
                   
                   
               
             
             
               
                   
                 FPA 
                 4K × 4K pixels 
                 4K × 4K pixels 
               
               
                   
                   
                 pixel size: 10 μm 
                 pixel size: 10 μm 
               
               
                   
                 FOV (deg.) 
                 26 deg. × 26 deg. 
                 small 
               
               
                   
                   
                 (38 deg. diagonal) 
               
               
                   
                 Focal Length (inch) 
                 3.04 
                 15.2 
               
               
                   
                 Speed 
                 F/3.0 
                 F/3.0 
               
               
                   
                 Aperture (inch) 
                 1.015 
                 5.07 
               
               
                   
                 IFOV (μrad) 
                 130 
                 26 
               
               
                   
                 Waveband (μm) 
                 3.8-4.2 
                 3.8-4.2 
               
               
                   
                   
               
             
          
         
       
     
         [0019]    In one embodiment, for image motion or jitter compensation, lens elements  14 B and  14 C are de-centered orthogonal the optical axis AA to control the line of sight. This technique has certain limitations based on image quality as some coma may be introduced. In one embodiment, a stabilization system is provided to suppress image jitter. The stabilization system includes a mechanism for de-centering at least one lens in the first lens group or in the second lens group, or both, orthogonal to an optical axis of the optical system. 
         [0020]    TABLE 2 lists various de-centering values of lens elements  14 B and  14 C relative to optical axis AA and obtained associated stabilization motion and a resulting root mean square value of wave front error (RMS WFE) (e.g., at the wavelength centered around 3.9 μm). In one embodiment, an RMS WFE value or range of values defines the image quality of an optical system at a certain radiation wavelength or in a certain range of radiation wavelengths. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Decenter 
                 Stabilization 
                 RMS WFE 
                   
               
               
                 (in.) 
                 Motion (μrad) 
                 (wvs at 3.9 μm) 
                 Comment 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0 
                 0 
                 0.0121 
                 Nominal Design 
               
               
                 0.0054 
                 100 
                 0.0195 
                 Negligible Degradation 
               
               
                 0.0108 
                 200 
                 0.0329 
                 Practical Useful Limit 
               
               
                 0.0162 
                 300 
                 0.0474 
                 Strehl Ratio = 0.91 
               
               
                   
               
             
          
         
       
     
         [0021]    TABLE 2 shows, for example, that if lens elements  14 B and  14 C are not decentered (decenter is equal to zero), the stabilization motion is equal to zero. In this configuration, the RMS WFE value is approximately 0.012 at a radiation wavelength centered around 3.9 μm. This corresponds to the nominal design of lens system  10 . However, when the lens elements  14 B and  14 C are de-centered by about 0.0054 inch with a motion stabilization of 100 μrad, the RMS WFE increases slightly to about 0.0195 at a radiation wavelength centered around 3.9 μm. This shows a relatively negligible degradation of the wave front aberration. When the lens elements  14 B and  14 C are de-centered by about 0.0108 inch with a motion stabilization of 200 μrad, the RMS WFE further increases to about 0.0329 at a radiation wavelength centered around 3.9 μm. This corresponds to a practical useful limit in terms of wave front aberration. 
         [0022]    TABLE 2 shows, for example, that ±200 μrads of stabilization motion control has a modest impact on wave front error and the de-center motion is only about ±0.0108 inches. The image quality of the nominal optical design is excellent with typical polychromatic wave front errors being in the range between about 0.01 and about 0.02 at a radiation wavelength centered around 3.8 μm across the field of view. 
         [0023]    It may be desirable that one or both the WFOV and NFOV capabilities be pointable or steerable within a field of regard (FOR) that is larger than either. Alternatively, it may be desirable for the NFOV optical system to be steerable or pointable within a FOR that is approximately equal in size to the WFOV. 
         [0024]    For relatively large angle, e.g., about ±26 deg., line of sight (LOS) steering of both the WFOV and NFOV paths within a FOR, a low weight alternative to a classical Az-El gimbal has been selected. In one embodiment, two rotating grisms  20  are used to steer the LOS. A grism is a prism having a diffraction grating on at least one of its faces. In one embodiment, the two counter-rotating grisms are made of silicon (Si). However, the two counter-rotating grisms can also be made from other optical materials, as needed. Although as described herein, the two prisms include diffraction grating surfaces, it is also contemplated that one or both of the prisms can have a diffraction grating surface. 
         [0025]      FIGS. 4 ,  5  and  6  show two rotating grisms  20  for steering the LOS into first and second lens groups  12  and  14 , according to one embodiment. The two counter-rotating prisms  20  are disposed in front of lens group  12 . Counter-rotating grisms  20  allow greater coverage of a viewed area by steering the ray paths from an angled direction to a parallel direction towards the imaging optics (lens groups  12  and  14 ).  FIGS. 4 ,  5  and  6  depict the position of the counter-rotating prisms  20  with respect to each other and their associated steering effect. 
         [0026]    In one embodiment, the diffraction grating on a surface of grism  20  is provided to cancel a linear component of the Si material dispersion. A non-linear dispersion component may cause lateral color dispersion in the imagery if the spectral band is very wide. 
         [0027]    TABLE 3 summarizes various parameters including the spectral band, the prism angle, the lateral color dispersion (PV Lateral Color) and grating period affecting the wave front error or the RMS WFE. For example, for a spectral band between about 3.4 μm and 4.2 μm, a prism angle of about 3.2 deg., a grating period of about 0.13 inch, and a PV lateral color of about 40 μrad, the obtained RMS WFE is approximately 0.15. For a spectral band between about 3.6 μm and 4.2 μm, a prism angle of about 3.2 deg., a grating period of about 0.14 inch, and a PV lateral color of about 21 μrad, the obtained RMS WFE is approximately 0.07. For a spectral band between about 3.8 μm and 4.2 μm, a prism angle of about 3.2 deg., a grating period of about 0.15 inch, and a PV lateral color of about 9 μrad, the obtained RMS WFE is approximately 0.03. In one embodiment, an RMS WFE of 0.03 provides a comfortable allocation, an RMS WFE of about 0.07 provides an acceptable allocation, and an RMS WFE of about 0.15 provides a degradation that is large. Therefore, the parameters are selected so as to achieve an RMS WFE smaller than 0.07 (an acceptable range). 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 Spectral 
                   
                 Grating 
                 PV Lateral 
                   
                   
               
               
                 Band 
                 Prism Angle 
                 Period 
                 Color 
                 RMS WFE 
                   
               
               
                 (μm) 
                 (deg.) 
                 (in) 
                 (μrad) 
                 (wvs) 
                 Comment 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 3.4-4.2 
                 3.2101 
                 0.1305 
                 40 
                 0.149 
                 Degradation 
               
               
                   
                   
                   
                   
                   
                 Large 
               
               
                 3.6-4.2 
                 3.2125 
                 0.1425 
                 21 
                 0.074 
                 Acceptable 
               
               
                 3.8-4.2 
                 3.2146 
                 0.1548 
                 9 
                 0.029 
                 Comfortable 
               
               
                   
                   
                   
                   
                   
                 Allocation 
               
               
                   
               
             
          
         
       
     
         [0028]    An extrapolation from TABLE 3 indicates that for ±26 deg. steering, for a spectral band between about 3.7 μm and 4.2 μm, and for a grating period of about 0.149 inch, the worst lateral color degradation of the polychromatic wave front error is about 0.045 at 3.8 μm. An RMS WFE of about 0.045 is below 0.07 and thus within an acceptable range. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                 Element 
                 RD 
                 Ad 
                 Ae 
                 Af 
                 Ag 
                 Thk 
                 Matl 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 Lens 12A 
                 14.751 
                   
                   
                   
                   
                 0.500 
                 Si 
               
               
                 2 
                   
                 88.600 
                   
                   
                   
                   
                 0.133 
                 air 
               
               
                 3 
                 Lens 12B 
                 −51.809 
                   
                   
                   
                   
                 0.350 
                 Ge 
               
               
                 4 
                   
                 −173.242 
                 −1.187E−05 
                 −2.208E−07 
                 −1.762E−10 
                 −1.127E−11 
                 0.310 
                 air 
               
               
                 5 
                 Lens 12C 
                 31.702 
                 −1.510E−03 
                 −5.310E−06 
                 −2.500E−06 
                 4.101E−07 
                 0.300 
                 Amtir1 
               
               
                 6 
                   
                 5.472 
                   
                   
                   
                   
                 5.619 
                 air 
               
               
                 7 
                 Lens 14A 
                 5.070 
                   
                   
                   
                   
                 0.500 
                 As2S3 
               
               
                 8 
                   
                 −11.326 
                 4.135E−03 
                 −2.890E−04 
                 2.304E−05 
                 −1.007E−06 
                 0.310 
                 air 
               
               
                 9 
                 Lens 14B 
                 −31.119 
                   
                   
                   
                   
                 0.200 
                 ZnS 
               
               
                 10 
                   
                 9.138 
                   
                   
                   
                   
                 0.736 
                 air 
               
               
                 11 
                 Lens 14C 
                 −24.773 
                   
                   
                   
                   
                 0.250 
                 Amtir1 
               
               
                 12 
                   
                 −8.013 
                   
                   
                   
                   
                 0.058 
                 air 
               
               
                 13 
                 Lens 14D 
                 3.077 
                   
                   
                   
                   
                 0.200 
                 CaF2 
               
               
                 14 
                   
                 1.865 
                   
                   
                   
                   
                 0.150 
                 air 
               
               
                 15 
                 Corrector 14E 
                 inf 
                   
                   
                   
                   
                 0.150 
                 Si 
               
               
                 16 
                   
                 inf 
                 −2.663E−04 
                 −1.166E−03 
                 5.967E−03 
                 −6.720E−03 
                 0.100 
                 air 
               
               
                 17 
                 Window 14F 
                 inf 
                   
                   
                   
                   
                 0.150 
                 Si 
               
               
                 18 
                   
                 inf 
                   
                   
                   
                   
                 0.100 
                 air 
               
               
                 19 
                 Stop 18 
                 inf 
                   
                   
                   
                   
                 2.402 
                 air 
               
               
                 20 
                 FPA 16 
                 inf 
                   
                   
                   
                   
                 n/a 
                 n/a 
               
               
                   
               
             
          
         
       
     
         [0029]    In one embodiment, the stop diameter is about 0.8 inch and the range of radiation wavelength is between about 3.6 μm and about 4.2 μm. 
         [0030]    TABLE 5 lists the various optical surfaces of optical system  10  and their respective radii of curvature (RD), aspheric coefficients (AD), (AE), (AF), and (AG), thickness (Thk), and type of material (Matl) when applicable, according to one embodiment. With his optical prescription, optical system  10  achieves an F-number or speed of about F/3 with a focal length of about 15 inches in the NFOV configuration and about 3 inches in the WFOV configuration, and a FOV of about 0.2 deg. in the NFOV configuration and a FOV of about 38 deg. in the WFOV configuration, as shown in TABLE 6. It should be noted that the total axial length of the optical system detailed in TABLE 5 is about 12.5 inches, while the focal length of the NFOV is about 15.2 inches. Whenever the physical length of an optical system is shorter than the effective focal length, the optical system is described as being telephoto. In this embodiment, the physical length is approximately 82% of the effective focal length. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                 Parameter 
                 NFOV 
                 WFOV 
                 units 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Aperture 
                 5.070 
                 1.014 
                 inch 
               
               
                   
                 Focal length 
                 15.208 
                 3.042 
                 inch 
               
               
                   
                 F-number 
                 3.00 
                 3.00 
                 n/a 
               
               
                   
                 FOV 
                 0.2 
                 38.0 
                 deg. 
               
               
                   
                   
               
             
          
         
       
     
         [0031]    It should be appreciated that in one embodiment, the drawings herein are drawn to scale (e.g., in correct proportion). However, it should also be appreciated that other proportions of parts may be employed in other embodiments. 
         [0032]    Although the inventive concept has been described in detail for the purpose of illustration based on various embodiments, it is to be understood that such detail is solely for that purpose and that the inventive concept is not limited to the disclosed embodiments, but, on the contrary, is intended to cover modifications and equivalent arrangements that are within the spirit and scope of the appended claims. For example, it is to be understood that the present disclosure contemplates that, to the extent possible, one or more features of any embodiment can be combined with one or more features of any other embodiment. 
         [0033]    Furthermore, since numerous modifications and changes will readily occur to those with skill in the art, it is not desired to limit the inventive concept to the exact construction and operation described herein. Accordingly, all suitable modifications and equivalents should be considered as falling within the spirit and scope of the present disclosure.