1. Field of the Invention
The present invention relates to systems and methods for volume holographic elements and gratings used to control the output wavelength of laser sources and apparatus for their fabrication.
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.
2. Background Art
Volume hologram reflection gratings have been shown to be an accurate and temperature-stable means of filtering a narrow passband of light from a broadband spectrum. This technology has been demonstrated in practical applications where narrow fill-width-at-half-maximum (FWHM) passbands ate required. Furthermore, such filters have arbitrarily selectable wavefront curvatures, center wavelengths, and output beam directions.
Photorefractive materials, such as LiNbO3 crystals and certain types of polymers and glasses, have been shown to be effective media for storing volume holographic gratings such as for optical filters or holographic optical memories with high diffraction efficiency and storage density. In addition, volume gratings Bragg-matched to reflect at normal incidence have been used successfully to stabilize and lock the wavelength of semiconductor laser diodes (U.S. Pat. No. 5,691,989).
In the prior art, a refractive element, such as a collimating lens, is used in conjunction with the volume holographic grating. Currently the holographic element is a separate element from the collimating lens, and is placed in the path of the already collimated beam. This requires an additional element to be assembled and aligned to the laser relative to the standard collimated but free-running laser. The alignment can be a problem because of asymmetric sensitivities of the devices. For example, the collimating lens may be rotation insensitive but translation sensitive. Conversely, the holographic element may be translation insensitive while being rotation sensitive. Matching and alignment then becomes a complex operation.
Different methods of recording or fabricating volume holographic gratings have also been used depending on the wavelength of the recording laser, determined by the material's sensitivity, and the wavelength used for readout, dictated by the application. In the conventional case, when the readout wavelength is at or near the recording wavelength, the recording geometry is essentially the same as that for the readout. In addition, gratings Bragg-matched to reflect at normal incidence have been recorded using a technique of writing from the side face in a transmission mode geometry with the correct incidence angle and a shorter wavelength of light, where the sensitivity of the material is much higher than at the, typically longer, readout wavelength (U.S. Pat. No. 5,491,570).
A laser utilizing feedback from a volume holographic grating integrated into a collimating lens used as a wavelength standard to lock the laser output wavelength to its desired value is described. This feedback is optical, with a volume hologram reflection grating integrated into a collimating lens used to generate optical feedback into the laser gain region.
In a first embodiment, the volume holographic grating consists of planar or curved surfaces of constant refractive index embedded throughout the volume of a collimating lens element. The lens can be a cylindrical or D type lens, or a spherical optic, or other form of collimating optic. Integration, of what in the prior art are two separate elements, reduces the complexity and cost of aligning and attaching the collimating and wavelength stabilizing components.
Another aspect of this invention is the method and apparatus of fabricating integrated elements. In one embodiment, the volume holographic grating is recorded through ordinary means in a slab of material. Then it is shaped into the desired shape of a lens. The recording can be with plane waves, to create planes of constant refractive index, or through the use of spherical or cylindrical wavefronts in order to create curved surfaces of constant refractive index.
In another embodiment, fabricating lens elements with integrated volume holographic gratings begins with a pre-formed material in which the recording is performed. The apparatus consists of a block of optical material within which one or more cavities are present to accept the unrecorded lenses. Within the cavity index-matching fluid is inserted to fill-in the space between the edge of the cavity and the lens. Due to the use of index matching fluid, it is not strictly necessary for the cavity shape to be the same as that of the lens element inserted into it. It must only be large enough for the lens to be able to be inserted and removed from the cavity. A usual holographic recording process is then used to expose the block with inserted lenses to record a grating with the desired characteristics.
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:
a and 3b are schematic depictions of planar and curved surfaces of constant refractive index within volume holographic grating lens elements.
a and 7b are schematic depictions of apparatus and methods of recording volume holographic gratings in lens elements.
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
Fabrication
The steps to fabricate a lens-shaped VHG element starting from a rectangular parallelepiped of material are shown in
An alternative means of fabricating lens elements with integrated VHG's is to begin with a pre-formed material in which the recording is performed.
Alternatively,
Thus, systems, methods and apparatus are described in conjunction with one or more specific embodiments. The invention is defined by the claims and their full scope of equivalents.
The applicant claims priority to provisional patent application No. 60/558,212 filed Mar. 30, 2004
Number | Date | Country | |
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60558212 | Mar 2004 | US |