Abstract:
The present invention relates to methods of measuring the optical characteristics of volume holographic gratings with high resolution and with a large spectral coverage using a spectrally broad band source in conjunction with instruments that measure the spectrum such as spectrometers, imaging spectrometers, and spectrum analyzers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present patent application is a continuation and claims the priority benefit of U.S. patent application No. 12/454,279 filed May 15, 2009, which claims the priority benefit of U.S. Provisional Pat. App. No. 61/127,799 filed May 5, 2008. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to methods of measuring the optical characteristics of volume holographic gratings with a large spectral coverage. 
     2. Notice of Material Subject to Copyright Protection 
     Portions of the disclosure of this patent document contain material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure as it appears in the Patent and Trademark Office file or records, but otherwise reserves all copyright rights whatsoever. 
     3. Description of the Related Art 
     Characteristics of volume holographic gratings are well known. When the Bragg condition λ r =2n(λ r )Λcos(θ r ), is satisfied the grating will diffract the incident beam into an output beam where λ r  is the readout wavelength in vacuum, n(λ r ) is the refractive index of the volume holographic grating element material at the readout wavelength, Λ is the grating spacing, and θ is the readout angle of incidence inside the material. The paper by H. Kogelnik (“Coupled wave theory for Thick Hologram gratings”, H. Kogelnik, The Bell System Tech. J. 48:9, 1969) provides details of volume holographic grating diffraction. For a simple uniform grating, the characteristics are completely determined by the thickness of the volume element, the refractive index modulation depth, the grating spacing, and the slant angle relative to the surface normal. For mass production of volume holographic grating elements it is desirable to measure these characteristics at the wafer level before dicing into final parts of smaller size. 
     U.S. Pat. No. 7,359,046 discloses methods to measure the optical characteristics of volume holographic gratings (VHG). The method relies on the concept of a collimated laser source  200  and a volume holographic grating  230  placed on a rotation stage  220  (FIG  1 .). By using a fixed laser wavelength whose value is below the normal incidence Bragg wavelength of the VHG, the optical characteristics of the VHG can be determined by rotating the VHG. High spectral and spatial resolution has been shown with this method. 
     The inconvenience of the method disclosed in U.S. Pat. No. 7,359,046 is that it requires single frequency laser sources. It is difficult and very expensive to find laser sources covering a wide spectral range such as for example UV to infrared (300 nm to 4000 nm). 
     SUMMARY OF THE INVENTION 
     The invention disclosed here teaches methods and apparatus to measure the characteristics of volume holographic gratings with high spectral and spatial resolution over a wide spectral range. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims and accompanying drawings where: 
         FIG. 1 : (prior art): Schematic of an apparatus to measure VHG with high spectral and spatial resolution. 
         FIG. 2 : (prior art) Spectra from Superluminescent LED (SLED), white light supercontinuum, incandescent light and various single mode lasers. 
         FIG. 3 : Schematic of an apparatus for measuring VHG with low spatial resolution and wide spectral coverage. 
         FIG. 4 : Schematic of an apparatus for measuring VHG with high spatial resolution and wide spectral coverage. 
         FIG. 5 : Schematic of an apparatus for measuring VHG with a step and scan giving medium spatial resolution and wide spectral coverage. 
         FIG. 6 : Schematic of another embodiment showing an apparatus for measuring VHG with step and scan giving medium spatial resolution and wide spectral coverage. 
         FIG. 7 : Schematic of another embodiment apparatus for measuring VHG with high spatial resolution and wide spectral coverage. 
         FIG. 8 : Schematic of another embodiment showing an apparatus for measuring VHG with step and scan giving medium spatial resolution, high signal to noise ratio and wide spectral coverage. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description of the present invention, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. 
     In a first embodiment illustrated by  FIG. 3 , the light from a broad-band source  300 , such as, but not limited, to a Superluminescent Light Emitting Diode (SLED), a high brightness lamp, or more recently, a supercontinuum white light source, is coupled into a fiber  310  which can be either single mode or multimode in the spectral range of interest.  FIG. 2  is a prior art diagram illustrating spectra from superluminescent LED (SLED), white light supercontinuum, incandescent light and various single mode lasers. The preferred embodiment is a supercontinuum white light source which has a continuous spectral coverage from approximately 400 nm to 2500 nm. The power level of the white light source is now reaching 6 W in a fiber core diameter of 3 micrometers. A collimating lens element  320  collimates the light from the broad-band source to provide a diffraction limited collimated beam. With a multimode fiber core, the diameter of the collimated beam needs to be large enough so that the angular divergence is smaller than the acceptance angle of the VHG element  330  near normal incidence. The VHG element  330  to be tested is placed in the path of the collimated light beam. This can be done for example, but not limited to, using the facet reflection of the VHG. The angle between the incident beam and the surface of the VHG element  330  is aligned to a pre-determined value. In one embodiment, a lens element  340  captures the light after the VHG element  330  and feeds it into a high resolution spectrometer or spectrum analyzer  350 . In this embodiment the spectrum analyzer  350  capture the whole beam of light which results in coarse spatial resolution. 
     In a variation of the previous embodiment, illustrated in  FIG. 4 , the light from a broadband source (e.g. supercontinuum source but not limited to)  400  is coupled into a fiber  410 . The light coming out of the fiber  410  is then collimated by lens element  420  and incident of the VHG element  430 . The lens element  440  and spectrometer  450  form an imaging spectrometer: at each pixel in the imaging spectrometer corresponds a spectral measurement. This method provides a high spatial resolution. The available high power from a supercontinuum source, for example, but not limited to, makes this method practical. 
     In another embodiment, illustrated in  FIG. 5 , the light from a broadband source (e.g. supercontinuum source but not limited to)  500  is coupled into a fiber  510 . The light coming out of the fiber  510  is then collimated by lens element  520  and incident of the VHG element  530 . The collimated beam size is made small and the VHG element  530  is spatially moved perpendicularly to the collimated beam in two dimensions so as to provide a spatial resolution of the order of the collimated beam spot size. The transmitted beam is fed into a high resolution spectrum analyzer  550  via lens element  540 . 
     In another embodiment, illustrated in  FIG. 6 , the light from a broadband source (e.g. supercontinuum source but not limited to) 6.00 is coupled into a fiber  610 . The light coming out of the fiber  620  is then collimated by lens element  620  and incident of the VHG element  630 . The collimated beam size is large, comparable in size with the VHG element  630 . An aperture  635  is placed after the VHG element  630 . The aperture  635  is translated perpendicularly to the collimated beam and in two dimensions to provide a selection of the area on the VHG element  630 . 
     The transmitted beam is fed into a high resolution spectrum analyzer  650  via lens element  640 . 
     In yet another embodiment, illustrated in  FIG. 7 , the light from a broadband source (e.g. supercontinuum source but not limited to)  700  is coupled into a fiber  710 . The light coming out of the fiber  720  is then collimated by lens element  720  and incident of the VHG element  730 . The collimated beam size is large, comparable in size with the VHG element  730 . A mechanically fixed aperture system  735  is placed after the VHG element  630 . The said fixed aperture system  735  provides a non-mechanical mean of switchable apertures such as provided by a Liquid Crystal modulator. The transmitted beam is fed into a high resolution spectrum analyzer  750  via lens element  740 . 
     In yet another embodiment, illustrated in  FIG. 8 , the light from a broadband source (e.g. supercontinuum source but not limited to)  800  is coupled into a fiber  810 . The light coming out of the fiber  810  is then collimated by lens element  820 . The collimated beam size is made small in comparison to the size of the reflective VHG element  830 . A beam-splitter  825  is placed between the collimating lens  820  and the reflective VHG element  830 . The VHG element is spatially moved perpendicularly to the collimated beam in two dimensions so as to provide a spatial resolution of the order of the collimated beam spot size. The antiparallel diffracted beam from the reflective VHG element  830  enters the beam splitter  825  again and a portion of it is reflected by the beam splitter  825  in to a high-resolution spectrometer  850  via the lens element  840 . 
     In all embodiments, the lens elements, fiber tip can be placed on translation stages so that diffraction limited collimation can be achieved for a wide spectral range. 
     In all embodiments, the apparatus can be calibrated by taking measurements without the sample in place. This will effectively eliminate measurement errors due to non-constant source spectra and wavelength dependencies of detectors or spectrometers. 
     It is to be understood that the invention is not limited to only work with light from a supercontinuum source, but of any sufficiently collimated source of broad-band electro-magnetic radiation, such as microwaves or terahertz waves, and is not limited to any specific range of the electro-magnetic spectrum. The invention is not limited to any specific material that contains volume holographic gratings, but applies to any and all materials that can store amplitude, phase, or some combination of the two. 
     Thus, systems and methods are described in conjunction with one or more specific embodiments. The invention is defined by the claims and their full scope of equivalents.