Abstract:
A virtual image phase array (VIPA) includes two parallel surfaces, a first highly-reflective surface with a highly-reflective coating, and a second partially-reflective surface. The first highly-reflective surface also requires an input zone with an anti-reflection coating, which abuts the highly-reflective coating, with a transition zone therebetween. Light enters the VIPA through the input zone, and reflects back and forth between the highly and partially reflective surfaces, gradually leaking out through the partially reflective surface. To minimize the transition zone and thereby minimize the input angle of incidence and maximize the number of reflections per unit of length, the substrate coated with the highly-reflective coating is subsequently polished at an acute angle resulting in the transition zone having the same sharp angle.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present invention claims priority from U.S. Provisional Patent Application No. 61/988,533 filed, May 5 2014, which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a virtual image phase array (VIPA), and in particular to a VIPA with an improved transition between input and reflective surfaces. 
       BACKGROUND OF THE INVENTION 
       [0003]    With reference to  FIG. 1 , a conventional VIPA  1  includes two parallel surfaces, a first highly-reflective surface  2 , which has a highly reflective coating  3  thereon, and a second partially-reflective surface  4  with a partially reflective coating  5  thereon. The first highly-reflective surface  2  also has an input, anti-reflection zone  6  with an anti-reflection coating  7 , which abuts the highly-reflective coating  3 , with a transition zone  8  therebetween. 
         [0004]    Light  9  entering the VIPA  1 , see  FIG. 2 , through the input zone  6 , reflects back and forth across a gap formed by substrate  10  between the highly and partially reflective surfaces  2  and  4 , respectively, gradually leaking out through the partially reflective surface  4 . Because the two reflective surfaces  2  and  4  are highly parallel, the output beams have a well-defined phase relationship, which enables the use of the VIPA  1  as a spectrometer, dispersion compensator, multiplexer/demultiplexer or filter. 
         [0005]    One of the keys to the operation of the VIPA  1  is a narrow transition zone  8  between the highly reflective surface  2  and the input zone  6 . Conventionally, the width of the transition zone  8  is controlled with some sort of mask during the deposition of the highly-reflective coating  3  and the anti-reflection (AR) coating  7  by a number of coating processes, e.g. electron beam evaporation, sputtering, etc. 
         [0006]    The mask could be a metal foil held in contact with the highly-reflective surface  2  during the deposition of the highly-reflective coating  3 , or a photoresist that is exposed and developed during assembly. After coating, with the highly-reflective coating  3 , the mask is removed, which may be a chemical removal process in the case of a photoresist. In either case, (or with any other masking technique), the width of the transition zone  8  is affected by the geometry of the mask, including the straightness of the mask, the thickness of the mask, the contact of the mask with the highly-reflective surface  2 , and the deposition process shadowing of the edge of the highly-reflective surface  2 . Typically, mechanical masking will result in a transition zone  8  with a width w of 50 μm or more. 
         [0007]    For the input light  9  to be efficiently coupled into the VIPA  1 , all of the light  9  must avoid the transition zone  8  during entry and after its first bounce from the partially reflective surface  4 . The width w of the transition zone  8  thus sets a minimum entrance angle from a normal to the input zone  6  into the VIPA  1 . Because VIPAs typically rely on a hundred or more bounces, a large transition zone  8  requires a large entrance angle resulting in a wider distance between bounces, and consequently increases the length of the VIPA  1  and the size of the associated optics in order to achieve the maximum spectral resolution of the device. Furthermore, the larger length increases the difficulty in manufacturing the VIPA  1 , resulting in a higher cost. 
         [0008]    An object of the present invention is to overcome the shortcomings of the prior art by providing a VIPA with a smaller transition zone to enable a smaller entrance angle for incoming light. 
       SUMMARY OF THE INVENTION 
       [0009]    Accordingly, the present invention relates to a virtual image phase array (VIPA) comprising: 
         [0010]    a transparent support substrate; 
         [0011]    a first reflective coating on a first reflective surface of the transparent support substrate; 
         [0012]    a second reflective surface separated by a gap from the first reflective coating; 
         [0013]    wherein the transparent support substrate and the first reflective coating include a beveled edge at an acute angle from the second reflective surface forming a narrow transition region at the edge of the first reflective coating between the first reflective coating and the beveled edge of the support substrate. 
         [0014]    Another aspect of the present invention relates to a method of fabricating a virtual image phase array (VIPA) comprising: 
         [0015]    providing a first support substrate with a first surface and an end; 
         [0016]    providing a first reflective coating on the first surface of the first support substrate; 
         [0017]    providing a second reflective coating substantially parallel to the first reflective coating with a gap therebetween; 
         [0018]    polishing the end of the first support substrate at an acute angle forming a beveled edge of the first support substrate and the first reflective coating, thereby forming a narrow transition region at the beveled edge of the first reflective coating between the first reflective coating and the beveled edge of the first support substrate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein: 
           [0020]      FIG. 1  illustrates a side view of a conventional VIPA; 
           [0021]      FIG. 2  illustrates a side view of the conventional VIPA of  FIG. 1  including a representation of light; 
           [0022]      FIG. 3  illustrates an isometric view of a VIPA in accordance with the present invention; 
           [0023]      FIG. 4  illustrates an isometric view of a VIPA in accordance with another embodiment of the present invention; 
           [0024]      FIG. 5  illustrates a side view of the VIPA of  FIG. 4  including a representation of light; 
           [0025]      FIG. 6   a  illustrates a side view of an initial step in a manufacturing process of a VIPA in accordance with the present invention; 
           [0026]      FIG. 6   b  illustrates a side view of a subsequent step in the manufacturing process of  FIG. 6   a;    
           [0027]      FIG. 6   c  illustrates a side view of a final step in the manufacturing process of  FIGS. 6   a  and  6   b;    
           [0028]      FIG. 7  illustrates a side view of a VIPA in accordance with another embodiment of the present invention; 
           [0029]      FIG. 8  illustrates a side view of a VIPA in accordance with another embodiment of the present invention; 
           [0030]      FIG. 9  illustrates a side view of a VIPA in accordance with another embodiment of the present invention; and 
           [0031]      FIG. 10  illustrates a side view of a VIPA in accordance with another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives and equivalents, as will be appreciated by those of skill in the art. 
         [0033]    With reference to  FIG. 3 , a virtual image phase array (VIPA)  11 , in accordance with an embodiment of the present invention, may be comprised of two or three pieces of glass (or other optical material), preferably optically contacted together. In particular, the VIPA  11  may comprise a generally rectangular main supporting substrate  12 , and a generally rectangular protective mounting substrate or backing plate  13 . A matching input section  21  may also be provided, as hereinafter described with reference to  FIG. 4 . The substrates  12  and  13  are generally rectangular for simplicity of manufacture and handling, but other shapes, e.g. round, oval etc., are possible and within the scope of the invention. 
         [0034]    The main supporting substrate  12 , e.g. piece of glass or other transparent material, preferably includes flat and parallel upper and lower (or first and second) opposed surfaces  14  and  15 , respectively, and accordingly has a uniform thickness. A transparent material, typically relates to a material suitably transparent at any wavelength of light, e.g. visible light, used in conjunction with the present invention. Flat preferably means with peak to valley flatness variations on the order of 10 nm for visible wavelengths of light, and uniform in thickness on the order of 1 to 2 nm RMS. The lower surface  15  may include a partially-reflective coating  16 , deposited thereon. Typically, the reflectivity of the lower surface  15  with the partially-reflective coating may be between 50% and 97%, preferably between 90% and 97%, more preferably between 93% and 97%, and ideally between 95% and 97% reflective. The upper surface  14  may include a highly-reflective coating  17  deposited thereon. Typically, the reflectivity of the upper surface  14  with the highly-reflective coating  17  is greater than 90%, preferably greater than 97%, and ideally greater than 99%. The mounting or blacking plate  13 , e.g. a piece of glass or other transparent material, may be similarly flat on at least a contact surface  18 , which is optically contacted to the highly-reflective surface  14  of the supporting substrate  12  with the highly-reflective coating  17  therebetween. The partially-reflective and highly-reflective coatings  16  and  17 , respectively, may be interchanged, if desired. 
         [0035]    The reflective coatings  16  and  17  should to be very flat, and conformed to the upper and lower surfaces  14  and  15  of the main supporting substrate  12  during the fabrication method. The reflective coatings  16  and  17  may be a metal film rather than a dielectric stack, but practically, most high reflectors will be dielectric stacks. 
         [0036]    The VIPA  11  includes the highly-reflective coating  17  sandwiched between the supporting substrate  12  and the backing plate  13 . During manufacture, the highly reflective coating  17  is deposited on the main supporting substrate  12 , and the backing plate  13  is mounted on the highly-reflective coating  17 . Then, the end of the VIPA  11  is beveled by polishing, or other suitable process, the ends of the main supporting substrate  12 , the backing plate  13 , and the highly-reflective coating  17  forming a beveled, flat input edge  19  at an acute angle α from the lower and contact surfaces  15  and  18  facing substantially away from the lower surface  15 , preferably between 30° and 60°, more preferably between 40° and 50°, and ideally about 45° to define a sharp bevelled edge to the highly reflective coating  17 . Ideally, the entire end of the main supporting and mounting substrates  12  and  13 , along with the highly-reflective coating  17  are polished at the acute angle α, but less than the entire ends are possible, as long as the highly-reflective coating  17  and the surrounding area of the substrates  12  and  13  are beveled. 
         [0037]    If the highly-reflective coating  17  is several wavelengths thick, which is typical for multilayer high reflector coatings, and the input edge  19  of the VIPA  11  is polished at e.g. a 45° angle α, a transition region  20  between the full-width, highly-reflective coating  17  and the surface of the assembly, i.e. the input edge  19  of the supporting substrate  12 , will be approximately 0.5× to 2.0×, preferably 0.8× to 1.2×, and more preferably the same as the thickness of the highly-reflective coating  17 , and much narrower than conventionally masked coatings. Because the highly-reflective coating  17  is supported on both sides by the main supporting substrate  12  and the backing plate  13 , it is protected from delamination or chipping during the polishing process. The partially-reflective coating  16  may be deposited on the lower surface  15  prior to or subsequent the polishing step. 
         [0038]    The highly-reflective coating  17  may be applied to the lower surface  15 , and the partially-reflective coatings  16 , may be applied to the upper surface  14 , whereby the transition region  20  is in the partially-reflective coating  16  and the input light is initially incident upon the highly-reflective coating  17 , if desired. 
         [0039]    Ideally, the transition region  20  may be small enough, e.g. less than 10 μm, preferably less than 5 μm, that it is unlikely to have much adverse effect on the performance of the VIPA  11  because the input beam has some physical width that will override the effects of the transition region  20 . For multilayer dielectric coatings, a visible/near IR reflective coating is likely to be 3 to 4 times thicker than the vacuum wavelength of the light it is designed to reflect. Accordingly, a highly reflective coating  17  for light with a wavelength of 1 μm would be on the order of 3 to 4 μm thick, depending on material indices, resulting in a transition zone  20  of between 1.5 μm to 8 μm, preferably 2.5 μm to 4.8 μm, and more preferably 3 μm to 4 μm wide. 
         [0040]    The VIPA  11  is fully functional in this form, whereby the light input surface is the beveled end  19  of the main supporting substrate  12 ; however, the improved VIPA  11 ′, illustrated in  FIG. 4 , is somewhat more convenient to use when the matching input section  21 , e.g. piece of glass or other transparent material, such as a triangular prism, is provided. The matching input section  21  may comprise a beveled surface  22  matching and parallel to the input edge  19 , and an anti-reflection coating  23  on an outer surface  24  thereof, providing a more convenient light input surface for the VIPA  11 ′. The matching input section  21  is preferably optically contacted to the input edge  19  of the VIPA  11 ′, but could also be cemented or otherwise fixed in place. 
         [0041]    The angled matching surface  22  of the matching input section  21  may be polished to include a beveled angle α from the outer surface complimentary to the edge surface  19 , so that the input surface  24 , i.e. AR coating  23 , of the completed VIPA  11 ′ is ideally parallel to the upper and lower, i.e. highly-reflective and partially-reflective, surfaces,  14  and  15 , respectively. However, even if the beveled angle deviates by several degrees, the fundamental performance of the VIPA  11 ′ is not affected. The input surface  24  may also be polished and coated before or after connection of the matching input section  21  to the input edge  19  to obtain the desired angle, e.g. a so that the input surface  24  is parallel to the upper and lower surfaces  14  and  15 , respectively. Again, the partially-reflective coating  16  may be deposited before or after the mounting of the matching input section  21 . 
         [0042]    For handling reasons, the sharp edge  26  on the supporting substrate  12  at the partially-reflective coating  16  may be rounded to include a chamfered edge  27 , as illustrated in  FIG. 5 , to prevent the partially-reflective coating  16  from chipping or becoming damaged in some other way. In this embodiment, the matching section  21 , e.g. triangular prism, has a thickness in between the total thickness of the support substrate  12  and the backing plate  13 , i.e. greater than the thickness of the support substrate  12 , but less than the combined thickness, whereby the input surface  24  is offset downwardly from an outer surface of the backing plate  13  towards the highly-reflective surface  14  to protect the AR coating  23 . 
         [0043]      FIG. 5  illustrates the completed VIPA  11 ′ in use. Note that the critical interface at the edge surface  19  of the highly-reflective surface  14  is protected from the local environment. Moreover, input light  30  can be input from at input port P at a much smaller entrance angle, e.g. angle of incidence I from a normal to the input surface  24 , than the conventional VIPA  1  because the transition zone  20  is much smaller than that of the conventional VIPA  1 , whereby reflecting light will still avoid the transition zone  20  entering and upon reflection. Accordingly, the input angle I may be less than 5°, preferably less than 2°, more preferably less than 1°, and even more preferably between 0.25° and 0.75°. Light which is to be analyzed by the VIPA  11 ′ is collected by a system of lenses at input port P. The lenses are selected to deliver as much light as possible through the AR coated input surface  24 , for reflecting multiple times from the partially reflective coating  16 , while not intercepting the transition region  20  at the end of the highly-reflective coating  17 . It is advantageous for the angle of incidence I to be as small as possible, so the lenses should be selected to produce a minimum beam width, i.e. a beam waist, at the lower (e.g. partially-reflective) surface  15 , and the beam width at the upper (e.g. highly-reflective) surface  14  to be as small as possible consistent with the beam waist condition, the wavelength of light being observed, and the quality of the input beam. 
         [0044]    For a conventional transition zone width of 50 μm, a wavelength of input light of 532 nm, and an input beam with an f/# of 60, the minimum input angle would be about 1.3°, but with a transition zone width of 2 μm, as in the present invention, the minimum input angle I would be about 0.51°. Following the invention, the length of the VIPA  11  (or  11 ′) could be reduced by 2.5×. There is another effect from the reduced angle of incidence, i.e. the dispersion relation for the VIPA  11  has a strong angle dependence, so this reduction in incident angle I reduces the number of orders, which are significantly illuminated, by about a factor of 2. 
         [0045]    The present design has the advantage that the matching input section  21 , in particular the beveled and outer surfaces  22  and  24 , respectively, may be prepared separately from the main substrate  12  and backing plate  13 , and subsequently fixed together. However, it is also possible to form a VIPA  31 , see  FIGS. 6   a  to  6   c , using a slightly more integrated method. In particular, the outer surfaces of a mounting substrate or backing plate  33 ′ and a matching input section  41 ′ may be polished after the matching input section  41  is optically contacted to the edge surface  19  of the main substrate  12 . According to  FIG. 6   a , a relatively rough outer-surfaced backing plate  33  may be mounted on the main substrate  12 , which may already have been treated on the upper and lower surfaces  14  and  15  with the highly and partially reflective coatings  17  and  16  forming the highly and partially reflective surfaces, respectively, as hereinbefore discussed. Subsequently, any flat (as hereinbefore defined) surface of a block of transparent material  41 , for example a rectangular block of glass, may be mounted on the beveled edge  19  of the VIPA  31  providing what will become the matching input section  41 ′. With reference to  FIG. 6   b , the upper surface of the combined VIPA  31  may then be formed by polishing the upper surface of the backing plate  33  and the block  41  forming the finished backing plate  33 ′ and the input matching section  41 ′ with a flat upper surface, in particular an input surface  42 , substantially parallel to the upper and lower surfaces  14  and  15 , respectively. Finally, with reference to  FIG. 6   c , an AR coating  43  may be applied to the entire upper surface of the combined VIPA  31 , including the backing plate  33 ′ and the input surface  42  of the input matching section  41 ′. The partially-reflective coating  16  may be applied before or after the polishing and the AR coating  43  steps. 
         [0046]    Another embodiment of the present invention, illustrated in  FIG. 7 , includes a VIPA  51 , which comprises only two supporting substrates of transparent material, e.g. pieces of glass, i.e. without a mounting substrate  13 . The main supporting substrate  12 , as hereinbefore described, includes the upper and lower surfaces  14  and  15 , with the highly- and partially reflective coatings  17  and  16 , respectively. During manufacture, the highly-reflective coating  17  (or the partially reflective coating  16 ) is deposited on the upper surface  14  of the main supporting substrate  12 , and the end of the coated substrate  12  is polished (or any other suitable process) to form the beveled edge  19 . Ideally, the entire end of the main substrate  12  from upper surface  14  to lower surface  15 , along with the highly-reflective coating  17 , are polished at the acute angle α, but less than the entire end is possible, as long as the highly-reflective coating  17  and the adjacent area of the substrate  12  are beveled. In addition, the VIPA  51  also includes a second matching input substrate or section  53 , e.g. glass or other transparent material. The matching input section  53  may be polished, or other suitable process, at one end  54  to include the complementary acute bevel angle α to the beveled edge  19  of the main supporting substrate  12 , whereby the input surface  56  on the matching input section  53  is parallel or substantially coplanar to the highly-reflective layer  14  on the first support substrate  12 . The acute bevel angle α is between the input surface  56  and the polished end  54 . An AR coating  57  may be applied to the input surface  56  prior to mounting the second input substrate  53  to the first supporting substrate  12  or after. The partially-reflective coating  16  may be deposited on the lower surface  15  of the main supporting substrate  12  at any time before or after the polishing step, and before or after the beveled end  54  of the matching input section  53  is mounted on the beveled edge  19 . 
         [0047]    Unfortunately, in the aforementioned embodiment, the highly-reflective coating  17  may not be protected by a mounting plate, and as a result is much more vulnerable to delamination or chipping. In addition, the sharp vertex  58  of the second input substrate  53  may potentially become the defining element in the transition zone  20  if it is chipped or damaged. 
         [0048]    The problem of edge chips in the matching input section  53  may be ameliorated by chamfering the acute edge  58 , and slightly offsetting the input surface  56  upwardly from the upper (highly-reflective) surface  14  during the mounting step, e.g. so that the upper (highly-reflective) surface  14  may be parallel, but in between, the input surface  56  and the lower (partially-reflective) surface  15 , with the input surface  56  and anti-reflection coating  57  overlapping and protecting the transition region  20 , as illustrated in  FIG. 8 . 
         [0049]    Polishing at acute angles α other than 45° is also within the scope of this invention. Polishing at higher angles will reduce the width of the transition region  20 , but is likely to increase the losses from the interface of the beveled surfaces  19 / 54  (or  19 / 22 ) between the input substrate  53  (or  21  or  41 ′) and the main substrate  12 . These losses are due to the increasing angle of incidence which will result in increasing reflection losses even from very small refractive index differences. For example, for an incident angle at the interface of 60°, the reflection loss is about 0.025% for a 0.1% index difference, this increases to about 0.13% at 70°, and gets progressively worse for higher angles of incidence. At the extreme, an incident angle at the interface of 89°, will result in 20% reflection loss, whereas at 45°, this same index difference would produce a loss no worse than 1 ppm. 
         [0050]    With reference to  FIGS. 9 and 10 , rather than using a single, solid, consolidated optical element, with reflective coatings on opposite sides of a solid cavity or gap formed by main substrate  12 , as hereinbefore disclosed, a VIPA  61  may comprise a first substrate  62  including a lower (highly-reflective) surface  63  including a highly reflective coating  64  thereon, and a separate second substrate  66  including an upper (partially-reflective) surface  67  including a partially-reflective coating  68  thereon with an air (or other suitable transparent gas, fluid or solid material) gap  69  between the two coatings  64  and  68 . The first substrate  62  includes a beveled, e.g. polished or other suitable process, edge  71  forming an acute angle α, as hereinbefore described, between both the highly-reflective and the partially-reflective surfaces  63  and  67 , respectively, and the beveled edge  71 . A matching third input substrate  72  includes a beveled end  73 , with an acute bevel angle α between an outer surface  74  and the beveled end  73 , complementary to and mounted on the beveled edge  71  of the first substrate  62 , whereby an input surface  76  on a lower surface of the matching third input substrate  72  may be parallel and substantially coplanar with the lower (highly-reflective) surface  63  and the highly-reflective coating  64  on the first substrate  62 . An AR coating  77  may be applied to the input surface  76  prior to mounting the third input substrate  72  to the first substrate  62  or after. 
         [0051]    During manufacture, the highly-reflective coating  64  may be deposited on the first substrate  62 , and an end of the coated first substrate  62  may be polished (or other suitable process) to form the beveled edge  71  at angle α, as hereinbefore defined. The matching third substrate  72  is formed, e.g. polished or other suitable process, with the beveled end  73 , outer surface  74 , and input surface  76 . The matching third input substrate  72  may then be mounted on the first substrate  62  with the beveled end  73  on the beveled edge  71 . Before, after or during these steps, the partially-reflective coating  68  is deposited on the second substrate  66 , and the two coated substrates  62  and  66  are fixed, spaced apart with only the air gap  69  therebetween, by an external frame or jig. 
         [0052]    This configuration is slightly more complicated because after fabrication, of the first and second coated substrates  62  and  66 , it is necessary to maintain their highly parallel alignment (&lt;0.1 μRad). As with the solid configuration, the width of the transition region  80  is a key determinant of the minimum angle of incidence, and hence the length of the VIPA  61 . 
         [0053]    In another exemplary embodiment illustrated in  FIG. 10 , a VIPA  81  comprises a first substrate  82  including a lower (e.g. highly-reflective) surface  83  including a highly-reflective coating  84 , and a separate second substrate  86  including an upper (e.g. partially-reflective) surface  87  with a partially-reflective coating  88  with an air (or other suitable transparent gas, fluid or solid material) cavity or gap  89  between the two coatings  84  and  88 . The first substrate  82  includes an inverted beveled edge  91  with the beveled edge  91  facing downwardly toward the partially-reflective surface  87 , i.e. forming an acute angle α, as hereinbefore defined, with the upper (uncoated) surface  92  of the first substrate  82 , an obtuse angle β, (for example 90+α) with the lower (e.g. highly-reflective) surface  83 , and an acute angle with the partially-reflective surface  87 , e.g. α when the upper surface  92  is parallel with the upper (e.g. partially-reflective) surface  87 . The refractive index of the first substrate  82  and the angles α and β are selected such that the input light travelling from input port P through air or other suitable atmosphere, through the beveled end of the first substrate  82  with a higher refractive index than air, and then out the beveled edge  91  of the first substrate  82  into the air gap  89 , will be refracted twice and directed onto the upper (e.g. partially reflective) surface  86  and into the VIPA cavity  89  at a desired input angle, e.g. with an angle of incidence of less than 5°, preferably less than 1°, and more preferably between 0.25° and 0.075°. 
         [0054]    Throughout the description, we have referred to various polished surfaces which are then coated to minimize reflection or to provide high reflectivity or partial reflectivity. The actual means of creating the surfaces is not critical to the function of the device, and although polishing is a common means of producing the required surface quality, other methods could be used, for example, cleaving of crystalline substrates. Similarly, typical optical reflectors are created by applying thin coatings to previously polished substrates, but the method of production of the coatings is not critical for the VIPA process. 
         [0055]    The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.