MEASUREMENT OF VOLUME HOLOGRAPHIC GRATINGS
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 Oflice 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 7»,:2n(7»,)Acos(0,), is satisfied the grating Will diffract the incident beam into an output beam Where 7», is the readout Wavelength in vacuum, n(7»,) is the refractive index of the volume holographic grating element material at the readout Wavelength, A is the grating spacing, and 0 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).
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.
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 cor