Volume-phase holographic diffraction grating optimized for the ultraviolet spectral region

A volume-phase holographic diffraction grating device is provided that can be optimized for use with very short wavelength light, such as light in the ultraviolet (UV) spectral region. The device comprises a cover and a substrate, both formed of a glass material. A layer of gelatin material is disposed between the cover and the substrate member, and has holographically-formed varying indexes of refraction formed therein to set up the interference pattern. The gelatin material has a thickness between 0.5 and 1 micron that makes it suitable for diffracting light in the UV spectral region, long-lived, and very efficient.

BACKGROUND OF THE INVENTION

The present invention relates to diffraction devices used in optical systems and devices, and more particularly to a volume-phase holographic diffraction device designed for performance in short-wavelength (i.e., ultraviolet) spectral regions.

Optical sensors often rely on information gathered from certain bands of light reflected or scattered from a material or object under investigation. In particular, energy associated with Raman scattering in the ultraviolet (UV) spectral region may be of interest in identifying certain materials. Because of the intrinsically weak nature of the signals on which these devices operate, the devices used to detect these weak signals should be as optically efficient as possible. Moreover, in a spectrograph, the light dispersion element (in many cases, a diffraction grating), is the major source of light loss. Other surfaces used to direct or otherwise modify the incoming signal add to the noise (from scattering) and attenuate the strength of the optical beam (from absorption). Thus, it is desirable to optimize the efficiency of each component in the spectrograph, and in particular the efficiency of the diffraction grating.

Another important factor in the design of an optical element, such as a diffraction grating, is its aging parameters. The materials used in fabricating the diffraction grating may break down over time from exposure to light, and may do so faster if the material interacts with the light in certain ways.

Conventional design philosophy for volume-phase holographic diffraction gratings has been that a gelatin material would not be suitable for a grating that is to operate on very short wavelengths of light. Many experts in the field believe that the gelatin material would absorb too much of the incoming light (and would thus be unsuitable in this wavelength regime) and would decay quickly. To the contrary, it has been discovered and as presented hereinafter, that a diffraction grating using a gelatin material can be optimized for operation on short wavelength light, such as light in the UV spectral region.

SUMMARY OF THE INVENTION

Briefly, a volume-phase holographic diffraction grating device is provided that can be optimized for use with very short wavelength light, such as light in the ultraviolet (UV) spectral region. The device comprises a cover and a substrate, both formed of a glass material. A layer of gelatin material is disposed between the cover and the substrate member, and has holographically-formed varying indexes of refraction formed therein to disperse and diffract incoming light. The gelatin material has a thickness between 0.5 and 1 micron that makes it suitable for diffracting light in the UV spectral region and very efficient. This grating design does not suffer from aging degradations.

DETAILED DESCRIPTION

Referring first toFIGS. 1A and 11B, the volume-phase diffraction grating is shown generally at reference numeral10. The grating10comprises a first (substrate) layer or member20and second (cover) layer or member30and a frame assembly40sandwiched therebetween. While the substrate20and cover30are shown as being generally rectangular, they make take on any shape, and are formed of UV-grade fused silica or other suitable glass material. The substrate20and cover30are at least ¼-wave (HeNe) flat (a flatness measure known in the art), for example, and are glued to each other around the frame assembly40with a suitable adhesive.

An anti-reflection coating may be applied to air-glass interfaces of the substrate20and cover30. For example, the anti-reflective coating should provide 99.5% or better transmissivity in the wavelength region of interest. A narrow-band or broad-band anti-reflection coating may be used depending on the application. Other coatings directed to other functions such as scratch resistance or to facilitate cleaning of the grating without affecting its performance may also be applied to the outer surfaces as desired.

Turning toFIGS. 2 and 3, the frame assembly40is shown in more detail. The frame assembly40comprises a frame44surrounding the periphery of a dichromated gelatin layer42. The frame44is formed of UV-grade fused silica or another appropriate material. On the outside of the frame44there is an adhesive material46that is used to hermetically seal the dichromated gelatin layer42without making contact with the gelatin layer42.

The dichromated gelatin layer42must have the chromium (dichromate) removed by a suitable chemical process once the gel has set in order to minimize the scatter it creates. An interference pattern is set up and causes varying indexes of refraction within the hologram created in the gelatin layer42. The number of “lines” associated with the hologram formed in the gelatin layer42is user-selectable and depends on the particular application of the grating. For example, the number of lines can range from small (e.g., 300) to reasonably large (6000 or more). Techniques for forming the hologram in a region of the gelatin layer42are well known in the art. Extremely high resolution may be achieved with a two pass-system where there is a mirror incorporated in the optical path.

FIG. 3shows that the frame member44serves as a stand-off between the gelatin layer42and the cover30. There is an air gap volume region60between the substrate20and the cover30in which the gelatin layer42is deposited and allowed to set on a surface of the substrate20. The gelatin layer42does not completely fill the air gap volume region60. The air gap60is necessary because the gelatin layer has such a small thickness it could not withstand the pressure of being sandwiched directly between the substrate20and cover30. The frame member44also serves to physically isolate the adhesive46from the gelatin layer42so that the adhesive46does not contaminate the gelatin layer42and degrade performance of the grating. There may be an anti-reflective coating applied to the surface of the cover30that makes contact with the air gap60. The adhesive46on the outside of the frame member44also adheres the substrate20to the cover30.

Generally, the transmission of the grating decreases with increasing thickness of the gelatin layer42. In addition, the sensitivity of the grating increases with increasing thickness; both relationships are wavelength dependent. It is important that the layer42have a very uniform thickness to minimize variation in performance in different areas of the grating. The dimensions of the grating may vary depending on the application. For example, the dichromated gelatin layer42may have a thickness in the range of approximately 0.5 to 1.0 microns to operate on unpolarized light in the 250–280 nm (UV) spectral region. In addition, the grating may be at least 110 mm square, having a clear aperture of at least 102 mm square.

The following sets forth the basis for determining the proper (ideal) thickness of the gelatin layer42for a volume-phase holographic grating (VPHG). For monochromatic light passing through a single material, the amount of transmission T is given by:

Through experimentation, relevant values for these parameters are shown in Table 1.

The grating equation describes the behavior of the VPHG:

Equation 2 indicates that the proper blaze angle θ for a 4000 lines/mm grating blazed for a wavelength of 260 nm in the first order is 23.°0578178, a value well within an allowed range of 5–30° as shown in Table 2. These parameters fix the blaze angle; the complete range of blaze angles considered here is 22.°619838–25.°907725 (a maximum Δθ of 3.°0287861) corresponding to wavelengths between 250–284 nm.

Once the blaze angle θ is known, the efficiency η of the grating is

η=sin2⁡(π⁢⁢Δ⁢⁢nxλ⁢⁢cos⁢⁢θ),(3)
in which Δn is the modulation in refractive index, x is the thickness of the gelatin layer, λ is the wavelength of interest, and θ is the angle at which the incoming light strikes the gelatin (i.e., the blaze angle; see Kogelnick, H., 1969. Bell Systems Technical Journal, Volume 48, pp. 2909, ff.). If Δn=0.14 (from Table 1) and the 4000 lines/mm grating is blazed for 260 nm in the first order, the efficiency of the grating as a function of the gelatin thickness is

The ideal gelatin thickness is the thickness that simultaneously minimizes absorption and scattering (Equation 1) and maximizes efficiency (Equation 3). The performance of the grating is the product of these two equations, defined as

The relation given by Equation 5 is shown by the curve inFIG. 4(each peak represents successive orders of refraction; the left-most peak corresponds to the order m=1). To find the ideal gelatin thickness, we take the derivative of τ with respect to x,

Once this value is established, the expression for the maximum efficiency, τmax, is

For the mid-range (nominal) values μ=3326.7944 cm−1, λ=260 nm, Δn=0.14, m=1, n=1.3, and Γ=4000 mm−1, the blaze angle θ is 23.°578178 and the ideal gelatin thickness is 0.802 μm; this results in a maximum theoretical efficiency of 75.96%.

These results provide for the calculation of the ideal gelatin thickness for VPHGs in any given configuration (α=β and φ=0). Should dichromate be left in the gelatin layer after processing (set-up and dichromate removal), the grating will be less efficient than these results indicate since the dichromate acts as an additional scatterer/absorber for incoming photons.

One method of assembling the diffraction grating10is to deposit the gelatin material for the layer42onto the substrate20at the desired thickness. The gelatin is allowed to set, and the chromate is then removed. Next, the frame member44is attached to the substrate20circumscribing the gelatin layer42. Some of the gelatin material may be removed to make room for the frame member44on the substrate20. The adhesive46is disposed around the exterior surfaces of the frame member44. The cover30is then placed over the substrate-frame sub-assembly and adhered to the substrate30by the adhesive46.

Turning toFIG. 5, operation of the grating10will be described. Incoming (source) light hits the grating at the cover side of the grating. Some of the incoming light in the 1storder (as an example in this case) is reflected from the cover, and the rest continues through the gelatin layer to the substrate. The light that is transmitted by the substrate includes transmitted light in the 0thorder and transmitted light in the 1storder. There is no refracted light into higher (or other competing) or negative orders and reflected light only travels 180 degrees to the refracted light or to the transmitted light.

The diffraction grating device and related concepts may be used in many applications, including any remote-sensing and/or imaging application that incorporates a spectrograph designed and/or used in the far blue or UV wavelength regime. In addition, it may be useful in astrophysics-related equipment and spaced-based observation devices that use a spectrograph designed and/or used in the far blue or UV wavelength regime.

The system and methods described herein may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing embodiments are therefore to be considered in all respects illustrative and not meant to be limiting.