Optical: systems and elements – Diffraction – From grating
Reexamination Certificate
2000-03-28
2004-02-17
Chang, Audrey (Department: 2872)
Optical: systems and elements
Diffraction
From grating
C359S566000, C359S571000
Reexamination Certificate
active
06693745
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to light dispersing devices and, in particular, relates to a thermally compensated light dispersing device utilizing a diffraction grating.
2. Description of the Related Art
Dispersing devices are often used in science and industry to separate beams of electromagnetic radiation, or light beams, according to wavelength. In particular, such devices are adapted to receive a polychromatic input light beam having a relatively large combined spectral bandwidth and convert the input beam into a plurality of constituent output light beams having relatively narrow bandwidths. The output beams exit the dispersing device with wavelength dependent directions so as to allow subsequent processing of individual wavelength components of the input beam.
One type of light dispersing device is a diffraction grating. The typical diffraction grating comprises a planar substrate and a relatively thin contoured layer formed on the substrate such that a contoured outer surface of the contoured layer defines a plurality of narrow grooves or slits. In the case of a reflection type grating, the contoured layer is formed of material having a high reflectivity so that light impinging the contoured surface is reflected therefrom. In the case of a transmission type grating, the contoured surface is provided with a low reflectivity and the substrate is formed of light transparent material so that light impinging the contoured surface is transmitted through the grating.
Thus, when a polychromatic incident light beam having a generally planar wavefront impinges on the contoured surface of the typical grating, a wavelet having a spherical wavefront is emitted from each of the grooves of the grating. As the wavelets travel away from the grating, they overlap and interfere with each other so as to provide a plurality of substantially monochromatic diffracted beams having wavelength dependent directions. Since the spectral bandwidth of the output beams is dependent on the number of grooves of the grating and since the contoured surface of the typical grating usually comprises a high density of grooves per unit length, the output beams are often provided with relatively narrow spectral bandwidths.
Mathematically, the dispersing characteristics of the diffraction grating are defined by the equation
s
(sin &thgr;
i
±sin &thgr;
m
)=
m&lgr;
(1)
wherein s is the groove spacing of the grating, &lgr; is the wavelength of the output beam, &thgr;
i
is the incident angle of the input beam with respect to a line normal to the plane of the grating, &thgr;
m
is the outgoing or diffraction angle of each output beam with respect to the line normal to the grating, m, otherwise known as the order of interference, can take any integer value, and wherein the plus sign applies to reflection-type gratings and the minus sign applies to transmission type gratings. Furthermore, the wavelength &lgr; of the output beam is defined by the equation
λ
=
λ
f
n
medium
(
2
)
wherein &lgr;
f
, otherwise known as the free space wavelength, is the wavelength of the diffracted beam in a vacuum, and n
medium
is the index of refraction of the medium in which the diffracted beam is traveling.
However, diffraction gratings known in the art often provide unstable dispersing characteristics that vary in response to a changing temperature. In particular, the groove spacing s is often affected by a change in temperature due primarily to the substrate of the grating having a non-zero coefficient of thermal expansion (CTE). Because the groove spacing is defined by the shape of the contoured surface and because the contoured layer is physically in contact with the substrate, the groove spacing s can change in response to expansion or contraction of the substrate. Since the groove spacing affects the dispersive characteristics of the grating according to equation (1), the dispersing characteristics of the typical diffraction grating may vary in a temperature dependent manner.
Another problem associated with typical diffraction gratings is that it is usually difficult to realize their theoretical maximum efficiency. In particular, the efficiency of a reflection-type grating is increased when the input and output beams are substantially aligned with each other. However, additional constraints imposed by the necessity of positioning additional elements adjacent the grating require the output beams to be angularly separated from the input beam with relatively large angles defined therebetween.
For example, in some configurations, a lens is often required to be positioned in the path of the input beam prior to the grating so as to collimate the input beam. Furthermore, spatial constraints may require the input lens to be positioned adjacent the grating. To prevent the input lens from distorting the output beams, the output beams are required to define relatively large angles with respect to the input beam. Consequently, the typical grating often provides less than optimal performance.
A prism is another type of dispersing device that is often used to extract narrow-band light beams from a polychromatic input beam. The typical prism utilizes non-parallel refracting input and output surfaces in conjunction with a transparent medium having a wavelength dependent index of refraction to provide the desired dispersing characteristics. Light entering and exiting each refracting surface of the prism is refracted according to Snell's Law of refraction which can be expressed mathematically by the equation:
n
i
sin &thgr;
i
=n
r
sin &thgr;
r
(3)
wherein n
i
is the index of refraction of an incident medium, &thgr;
i
is the incident angle of the incident beam defined with respect to a line perpendicular to the refracting surface, n
r
is the index of refraction of the refracting medium adjacent the incident medium, and &thgr;
r
is the refracted angle of the refracted beam. Because the index of refraction of the prism is dependent on the free space wavelength of light traveling therethrough, light exiting the output surface does so with a wavelength dependent direction. However, since the index of refraction of the typical prism usually varies with a change in temperature in a substantial manner, the dispersing characteristics of the prism are also relatively unstable and vary in response to a change in temperature.
Another type of dispersing device combines a diffraction grating and a prism so as to form what is otherwise known as a “grism”. One advantage of the grism is that it provides convenient mounting surfaces for facilitating alignment of the grism within an optical system. Furthermore, as described in U.S. Pat. No. 5,652,681 to Chen et al., the grism can be adapted to provide improved resolving power, such that the device is able to disperse finely spaced wavelength components with increased angular separation over a larger spectral range. However, because the groove spacing of the typical diffraction grating and the index of refraction of the typical prism are both adversely affected by a change in temperature, the dispersive characteristics of the typical grism are especially sensitive to a change in temperature.
From the foregoing, therefore, it will be appreciated that there is a need for an improved light dispersing device with dispersing characteristics that are more stable with respect to temperature. Furthermore, there is a need for a dispersing device with increased throughput efficiency.
SUMMARY OF THE INVENTION
The aforementioned needs are satisfied by the present invention which, in one aspect, comprises a dispersing apparatus having an input face, a diffracting element, and an output face. The input face refracts light at angles which vary with temperature. The diffracting element compensates for these angular variations such that light exits the dispersing apparatus refracted at angles that are substantially independent of the angular variations of the input face.
In one embodiment, light entering the di
DeBoynton William Leon
Filhaber John F.
Kondis John P.
Scott Bradley A.
Chang Audrey
Corning Incorporated
Knobbe Martens Olson & Bear LLP
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