Abstract:
A method and apparatus involve: routing first radiation and second radiation respectively having first and second wavelengths that are different along respective first and second optical paths; reflecting the first radiation with an optical component as the first radiation is traveling along the first optical path; and reflecting the second radiation with the optical component as the second radiation is traveling along the second optical path, the optical component causing a first optical path length traveled by the first radiation along the first optical path from arrival at to departure from the optical component to be shorter than a second optical path length traveled by the second radiation along the second optical path from arrival at to departure from the optical component.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates in general to optical systems and, more particularly, to techniques that compensate for dispersion in optical systems. 
       BACKGROUND 
       [0002]    In an optical system, a transmissive optical element such as a lens or window will often exhibit slightly different indexes of refraction to respective different wavelengths of radiation. This is commonly referred to as the dispersion characteristic of the component. When different wavelengths pass through the optical component, they can be displaced relative to each other. As a result, they will typically have different optical path lengths within the component, and will ultimately focus at different locations along an optical axis. If the different wavelengths pass through multiple optical components, the effects of different optical path lengths for respective wavelengths can be cumulative. 
         [0003]    To design a viable optical system that functions at two or more wavelengths, it is generally necessary to minimize dispersive effects. However, minimizing dispersive effects adds a significant level of complexity to the design process, particularly as the system&#39;s range of operational wavelengths increases. Not only must suitable optical materials such as glasses be selected for the various optical components, but suitable shapes, a suitable order, and suitable positions must also be determined. The materials, shapes, order and positions of the optical components interact, and thus a designer usually must spend a great deal of time balancing all of these interacting factors in order to find an acceptable combination. Although pre-existing techniques of this general type have been generally adequate for their intended purposes, they have not been satisfactory in all respects. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    A better understanding of the present invention will be realized from the detailed description that follows, taken in conjunction with the accompanying drawings, in which: 
           [0005]      FIG. 1  is a diagrammatic view of an apparatus that is an optical system, and that includes an optical component. 
           [0006]      FIG. 2  is a diagrammatic fragmentary sectional side view showing the optical component of  FIG. 1  in an enlarged scale, and providing additional detail regarding its structure. 
           [0007]      FIG. 3  is a diagrammatic fragmentary sectional side view of an optical component that is an alternative embodiment of the optical component of  FIGS. 1 and 2 . 
           [0008]      FIG. 4  is a diagrammatic fragmentary sectional side view of an optical component that is an alternative embodiment of the optical component of  FIG. 3 . 
           [0009]      FIG. 5  is a diagrammatic fragmentary side view of an optical component that is a further alternative embodiment of the optical component of  FIG. 3 . 
           [0010]      FIG. 6  is a diagrammatic fragmentary side view of an optical component that is a further alternative embodiment of the optical component of  FIG. 1 . 
           [0011]      FIG. 7  is a diagrammatic fragmentary side view of an optical component that is yet another alternative embodiment of the optical component of  FIG. 1 . 
           [0012]      FIG. 8  is a diagrammatic view of an apparatus in the form of an optical system that is an alternative embodiment of the optical system of  FIG. 1 , and that includes an optical component. 
           [0013]      FIG. 9  is a diagrammatic fragmentary sectional side view showing the optical component of  FIG. 8  in an enlarged scale, and providing additional detail about its structure. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]      FIG. 1  is a diagrammatic view of an apparatus that is an optical system  10 . The following discussion of the optical system  10  mentions various thicknesses, and in this regard it is relevant to understand the difference between physical thickness and optical thickness. More specifically, optical thickness “t” is defined as refractive index “n” multiplied by the actual physical thickness “d”, or in other words t=nd. Thus, if two different materials have identical physical thicknesses but different refractive indices, they will have different optical thicknesses. Conversely, if two materials have identical optical thicknesses and different refractive indices, they will have different physical thicknesses. 
         [0015]    Focusing specifically now on the optical system  10 , an element  12  that is a piece of fused silica has a physical thickness of 1 cm. Two beams  16  and  17  of laser light pass through the element  12 , and have different wavelengths. In particular, the beam  16  has a wavelength of 1064 nm, and the beam  17  has a wavelength of 1542 nm. These two specific wavelengths are mentioned here only for the sake of example. The beams  16  and  17  could alternatively have other wavelengths, provided the wavelength of beam  16  is different from the wavelength of beam  17 . The beams  16  and  17  would typically be coincident but, for purposes of clarity in explaining the optical system  10 , they are depicted in  FIG. 1  as being slightly spaced. 
         [0016]    The element  12  of fused silica exhibits slightly different indexes of refraction with respect to the respective wavelengths of each of the beams  16  and  17 . In particular, the element  12  exhibits a refractive index of 1.44963 with respect to the 1064 nm wavelength of beam  16 , and exhibits a refractive index of 1.44412 with respect to the 1542 nm wavelength of beam  17 . Due to this slight difference in refractive indexes, the beams  16  and  17  will be displaced with respect to each other as they travel through the element  12 , thereby causing the optical path length traveled by the beam  16  within element  12  to be about 55.1 μm longer than the optical path length traveled by beam  17  within element  12 . As a result, the beams  16  and  17  will ultimately focus at different points along an optical axis. 
         [0017]    The fact that different wavelengths are influenced differently by the element  12  is commonly referred to as the dispersion characteristic of the element  12 . For simplicity,  FIG. 1  shows only a single element  12  with a dispersion characteristic that exerts a different influence on each of the beams  16  and  17 . However, the element  12  is just one example of optics having a dispersion characteristic. In place of the element  12 , some other optical component, or a combination of optical components, could produce dispersion. 
         [0018]    The optical system  10  includes another optical component  21  that is provided specifically to counteract and thus compensate for the dispersive effect of the element  12 . In more detail, and as noted above, the optical path length traveled by beam  16  within element  12  is about 55.1 μm longer than the optical path length traveled by beam  17  within element  12 . The optical component  21  introduces an optical path length differential that is approximately equal and opposite to that introduced by the element  12 . Thus, the optical path length traveled by beam  17  within optical component  21  is about 55.1 μm longer than the optical path length traveled by beam  16  within the optical component. 
         [0019]    For clarity, the optical component  21  is not shown to scale in  FIG. 1 . The optical component  21  includes a substrate  24  that, in the disclosed embodiment, is made of an optical glass. More particularly, the substrate  24  in  FIG. 1  is made from a material that is commonly known as BK7 glass, and that can be obtained commercially from a number of sources, one of which is Red Optronics of Mountain View, Calif. Alternatively, however, the substrate  24  could be made of any other suitable material. 
         [0020]    The optical component  21  also includes a reflective section  26  that is provided on one side surface of the glass substrate  24 , and a further reflective section  27  that is provided on a side of the reflective section  26  opposite from substrate  24 . The reflective section  26  is reflective to the beam  17 , or in other words is reflective to radiation within a range of wavelengths that includes the wavelength 1542 nm. The reflective section  27  passes the beam  17 , and reflects the beam  16 . In other words, the reflective section  27  is transmissive to radiation within a range of wavelengths that includes the wavelength 1542 nm, and is reflective to radiation within a different range of wavelengths that includes the wavelength 1064 nm. The reflective section  27  has an optical thickness of approximately 27.55 μm, or in other words half of 55.1 μm. 
         [0021]    It will be noted from  FIG. 1  that, although the beam  16  is reflected by the reflective section  27 , the beam  17  passes through the reflective section  27 , is reflected by the reflective section  26 , and then passes back through the reflective section  27 . Thus, due to the fact that the beam  17  passes through the reflective section  27  twice, the optical path length of the beam  17  within the optical component  21  is about 55.1 μm longer than that for the beam  16 , thereby counteracting and compensating for the optical path length differential introduced by the element  12 . 
         [0022]      FIG. 2  is a diagrammatic fragmentary sectional side view showing the optical component  21  of  FIG. 1  in an enlarged scale, and providing additional detail about its structure. For clarity, the various portions of the optical component  21  are not shown to scale in  FIG. 2 . In  FIG. 2 , the reflective sections  26  and  27  are each implemented as a multi-layer thin-film optical filter. The reflective section  26  includes alternating layers of silicon dioxide (SiO 2 ) and tantalum oxide (Ta 2 O 5 ) that each have an optical thickness of 1 quarter-wave (QW) of the wavelength 1542 nm, or in other words an optical thickness that is 25% of 1542 nm, and thus 385.5 nm. Reflective section  26  includes 20 of the SiO 2  layers, and 20 of the Ta 2 O 5  layers. Similarly, the reflective section  27  includes alternating layers of SiO 2  and Ta 2 O 5  that each have an optical thickness of 1 QW of the wavelength 1064 nm, or in other words an optical thickness that is 25% of 1064, and thus 266 nm. Reflective section  27  includes 50 of the SiO 2  layers, and 50 of the Ta 2 O 5  layers. 
         [0023]    The optical component  21  further includes two thin layers  31  and  32  that are provided on the outer side of the reflective section  27 . Layers  31  and  32  are so thin that they are not shown separately in  FIG. 1 , but they are shown diagrammatically in  FIG. 2 . Layer  31  is a single layer of Ta 2 O 5  having an optical thickness of 1.3 QW of the wavelength 1542 nm. Layer  32  is a single layer of SiO 2  having an optical thickness of 0.7 QW of the wavelength 1542 nm. Absent the layers  31  and  32 , the reflective section  27  would reflect a small portion of the energy of the beam  17  having the wavelength of 1542 nm. The layers  31  and  32  serve to prevent any significant reflection of the radiation of beam  17 , so that virtually all the radiation of beam  17  enters the optical component  21  and travels through the reflective section  27  to the reflective section  26 . 
         [0024]    The specific materials and layers shown in  FIG. 2  represent only one possible configuration for the optical component  21 . Other configurations of materials and/or other layers could alternatively be utilized. For example, if the beams  16  and  17  of  FIG. 1  happened to have wavelengths different from the two exemplary wavelengths of 1064 nm and 1542 nm discussed above, the materials and/or structure of the optical component  21  would be adjusted so as to provide appropriate optical path-length compensation for the specific wavelengths involved. 
         [0025]    The particular construction shown in  FIG. 2  for the optical component  21  assumes that the beams  16  and  17  of  FIG. 1  each impinge on the optical component  21  at an angle of approximately 10° with respect to a not-illustrated reference line extending perpendicular to the layers in the optical component  21 . In some other application where radiation will impinge on the optical component  21  at a different angle, the combination of materials and/or layering can be adjusted to optimize performance for that angle. 
         [0026]    The specific construction shown in  FIG. 2  for the optical component  21  reduces the overall optical path length differential between beams  16  and  17  to about 0.3 μm. In order to reduce this differential to approximately zero, an additional layer can be added between the reflective sections  26  and  27 , and can have an optical thickness of 0.15 μm (0.3 μm divided by 2). In this regard,  FIG. 3  is a diagrammatic fragmentary sectional side view of an optical component  51  that is an alternative embodiment of the optical component  21  of  FIGS. 1 and 2 . Identical or equivalent portions are identified with the same reference numerals, and the following discussion will focus on the differences. For clarity, the optical component  51  is not shown to scale in  FIG. 3 . 
         [0027]    The optical component  51  is effectively identical to the optical component  21 , except that an additional layer  53  has been added between the reflective sections  26  and  27 , and serves as a spacer layer. The spacer layer  53  is a single layer of SiO 2  having an optical thickness of 0.1 QW of the wavelength 1542 nm. As noted above, if the optical component  51  is substituted for the optical component  21  in  FIG. 1 , the optical component  51  will almost completely counteract and compensate for the optical path length differential introduced between the beams  16  and  17  by the element  12 . 
         [0028]    In another variation, the optical thickness of the spacer layer  53  can be increased, and the number of pairs of alternating layers in the reflective section  27  can be decreased. As an example of this approach,  FIG. 4  is a diagrammatic fragmentary sectional side view of an optical component  61  that is an alternative embodiment of the optical component  51  of  FIG. 3 . Identical or equivalent portions are identified with the same reference numerals, and the following discussion focuses on the differences. For clarity, the optical component  61  is not shown to scale in  FIG. 4 . In the optical component  61 , a reflective section  63  has replaced the reflective section  27  of the optical component  51  in  FIG. 3 . The reflective section  63  differs from the reflective section  27  in that it has fewer pairs of alternating layers. In particular, the reflective section  63  has 30 pairs of alternating layers, rather than 50 pairs of alternating layers. More specifically, the reflective section  63  includes 30 layers of SiO 2  that alternate with 30 layers of Ta 2 O 5 , each of these 60 layers having an optical thickness of 1 QW of the wavelength 1064 nm. 
         [0029]    In the optical component  61  of  FIG. 4 , two spacer layers  67  and  68  have replaced the spacer layer  53  of the optical component  51  in  FIG. 3 . The spacer layer  67  is a single layer of SiO 2  having an optical thickness of 0.1 QW of the wavelength 1542 nm, and the spacer layer  68  is a single layer of SiO 2  having an optical thickness of 40 QW of the wavelength 1064 nm. Because the spacer layers  67  and  68  are each made of SiO 2  and physically engage each other, as a practical matter they collectively form only a single layer of SiO 2 . They are shown as separate layers in  FIG. 4  only to facilitate a clearer understanding of the optical component  61 . A broken line is provided between layers  67  and  68  to diagrammatically indicate that they are actually just different portions of a single layer. 
         [0030]      FIG. 5  is a diagrammatic fragmentary side view of an optical component  71  that is a further alternative embodiment of the optical component  51  of  FIG. 3 . Identical or equivalent parts are identified with the same reference numerals, and the following discussion focuses on the differences. For clarity, the optical component  71  is not shown to scale in  FIG. 5 . The optical component  71  of  FIG. 5  is intended for use in an application where a relatively large optical path length differential exists between the two radiation beams  16  and  17 . Because the optical path length differential is relatively large, and a relatively large optical path length correction is needed, a relatively large spacer layer  73  is provided between the reflective sections  26  and  27 , the spacer layer  73  having an optical thickness that is appropriate to obtain the desired optical path length correction. Due to the fact that the spacer layer  73  is relatively thick, the spacer layer  73  also serves as a substrate, and so the glass substrate shown at  24  in  FIG. 3  is omitted from the optical component  71  of  FIG. 5 . 
         [0031]      FIG. 6  is a diagrammatic fragmentary side view of an optical component  81  that is a further alternative embodiment of the optical component  21  of  FIG. 1 . Identical or equivalent portions are identified with the same reference numerals, and the following discussion focuses on the differences. For clarity, the optical component  81  is not shown to scale in  FIG. 6 . The optical component  81  includes the same glass substrate  24  and the same reflective sections  26  and  27  as the optical component  21 , but they are arranged differently. In particular, the reflective sections  26  and  27  are provided on opposite sides of the substrate  24 , with the reflective section  27  disposed between the reflective section  26  and the substrate  24 . Between the reflective section  27  and the substrate  24  there are two layers that are functionally equivalent to the layers  31  and  32  in  FIG. 3 , but have optical thicknesses different from those of the layers  31  and  32 , and are so thin that they are not visible in  FIG. 6 . The beam  16  travels through the substrate  24  to the reflective section  27 , where it is reflected and then travels back through the substrate  24 . The beam  17  travels through the substrate  24  and the reflective section  27  to the reflective section  26 , where it is reflected and travels back through the reflective section  27  and the substrate  24 . In the embodiment of  FIG. 6 , if the material of the substrate  24  happens to introduce any dispersive effect, the reflective sections  26  and  27  can be adjusted to compensate for that. 
         [0032]      FIG. 7  is a diagrammatic fragmentary side view of an optical component  91  that is yet another alternative embodiment of the optical component  21  of  FIG. 1 . Identical or equivalent portions are identified with the same reference numerals, and the following discussion focuses on the differences. For clarity, the optical component  91  is not shown to scale in  FIG. 3 . The optical component  91  includes the reflective section  27  of the optical component  21 , but the reflective section  26  is omitted, and the glass substrate  24  is replaced with a substrate  93  that is made of aluminum and that has a polished external surface  94  which is reflective to the 1542 nm wavelength of the beam  17 . The reflective section  27  is provided on the surface  94 . The substrate  93  could alternatively be made from some other type of metal, or from some other suitable material that can have a surface which is highly reflective to the relevant wavelength(s). As another alternative, the substrate  93  could be made from glass or some other non-reflective material, and could have on the surface  94  a thin layer of a metal or some other material that is highly reflective to the relevant wavelength(s). 
         [0033]    For simplicity, the foregoing discussion of  FIGS. 1-7  has been presented in the context of two specific wavelengths that are 1064 nm and 1542 nm. However, the beams  16  and  17  could alternatively involve mutually exclusive ranges of wavelengths that have 1064 nm and 1524 nm as respective center wavelengths. Moreover, the invention is not limited to two specific wavelengths, or even two ranges of wavelengths, but could instead involve three or more wavelengths, and/or three or more wavelength ranges. As one example,  FIG. 8  is a diagrammatic view of an apparatus in the form of an optical system  110  that is an alternative embodiment of the optical system  10  of  FIG. 1 . Identical or equivalent portions are identified with the same reference numerals, and the following discussion focuses on the differences. 
         [0034]    In the system  110  of  FIG. 8 , an optical element  112  has been substituted for the fused silica element  12  of the system  10  in  FIG. 1 . The optical element  112  is made of BK7 glass, and has a physical thickness of 1 cm. Three radiation beams  116 ,  117  and  118  of laser light propagate through the element  112 . The beams  116 - 118  would normally be coincident, but for clarity they are shown in  FIG. 8  as being slightly spaced. The beam  116  has a wavelength of 443 nm, the beam  117  has a wavelength of 539 nm, and the beam  118  has a wavelength of 653 nm. These three wavelengths correspond to the basic colors of red, green and blue. Alternatively, the beams  116 - 118  could each involve a range of wavelengths, where the respective indicated wavelength is the center wavelength of the range. The optical path length of the beam  116  within element  112  will be approximately 69.1 μm longer than the optical path length of beam  117  within the element, and the optical path length of beam  117  within the element will be approximately 46.5 μm longer than the optical path length of the beam  118  within the element. 
         [0035]    An optical component  121  influences the relative optical path lengths of the beams  116 ,  117  and  118  in a manner that is equal and opposite to the optical path length differential introduced by the element  112 . For clarity, the optical component  121  is not shown to scale in  FIG. 8 . The optical component  121  includes the glass substrate  24 , and three reflective sections  126 ,  127  and  128  provided on a side of the substrate  24  optically nearest the element  112 . The reflective section  128  reflects the beam  116 , and passes the beams  117  and  118 . The reflective section  128  has an optical thickness of 34.55 μm, which is half the optical path-length differential of 69.1 μm between the beams  116  and  117 . The reflective section  127  reflects the beam  117 , and passes the beam  118 . The reflective section  127  has an optical thickness of 23.25 μm, which is half the 46.5 micron optical path length differential between the beams  117  and  118 . The reflective section  126  reflects the beam  118 . 
         [0036]      FIG. 9  is a diagrammatic fragmentary sectional side view showing the optical component  121  of  FIG. 8  in an enlarged scale, and providing additional detail about its structure. For clarity, the optical component  121  is not shown to scale in  FIG. 9 . In  FIG. 9 , the reflective section  126  has 20 layers of SiO 2  that alternate with 20 layers of Ta 2 O 5 , where each of these 40 layers has an optical thickness of 1 QW of the wavelength of 653 nm. The reflective section  127  has 43 layers of SiO 2  that each have an optical thickness of 1.4 QW of wavelength 539 nm, and that alternate with 43 layers of Ta 2 O 5  that each have an optical thickness of 0.6 QW of wavelength 539 nm. The reflective section  128  has 79 layers of SiO 2  that each have an optical thickness of 1 QW of wavelength 443 nm, and that alternate with 79 layers of Ta 2 O 5  that each have an optical thickness of 1 QW of wavelength 443 nm. The reflective section  126  reflects a waveband of approximately 610 nm to 700 nm. The reflective section  127  reflects a waveband of approximately 500 nm to 600 nm while passing the waveband of approximately 610 nm to 700 nm. The reflective section  128  reflects a waveband of approximately 400 nm to 490 nm, while passing a waveband of approximately 500 nm to 700 nm. 
         [0037]    One practical application for the optical component  121  of  FIGS. 8 and 9  is a not-illustrated color projector of the type that has multiple interchangeable lens assemblies. Each lens assembly has a respective different focal length, and has different optical characteristics. One such optical characteristic is dispersion, which affects the location at which each wavelength focuses along an optical axis. Each of the multiple different lens assemblies could include a respective optical component of the general type shown at  121  in  FIGS. 8 and 9 , where the multiple reflective sections of each such component would be configured to effect the appropriate optical path length adjustments needed for that particular lens assembly. 
         [0038]    In each of the optical components  21 ,  51 ,  61 ,  71 ,  81 ,  91  and  121  respectively shown in  FIGS. 2 ,  3 ,  4 ,  5 ,  6 ,  7  and  9 , more than 99% of the energy of each wavelength will be reflected if the layers in the reflective sections are all of good quality. The theoretical performances are more than 99.9%. 
         [0039]    In the disclosed embodiments, the minimum optical path length differential that can reasonably be corrected is limited by the need to have a high degree of reflection for one wavelength range in each reflective section that must also pass one or more other wavelength ranges. As a reflective section gets thinner, its reflectivity decreases. For visible light, a reasonable minimum optical thickness is approximately 4 μm, depending on what is considered to be an acceptable loss for the particular application. 
         [0040]    Although selected embodiments have been illustrated and described in detail, it should be understood that a variety of substitutions and alterations are possible without departing from the spirit and scope of the present invention, as defined by the claims that follow.