Patent Application
20080137171
The present invention relates generally to optical technology, and more particularly to a phase mask that utilizes a continuous phase variation in an azimuthal direction for use in a coronagraph imaging system.
Detecting planets outside of the solar system is frequently complicated by the relative proximity of extrasolar planets to the stars they orbit. Since planets can be located only by the reflected light of their star, the star will be significantly brighter than the planet of interest, in some cases on the order of ten million times brighter. In light of the relative proximity of a planet to its star, it is necessary to attenuate the light of the star to obtain a useful image of the planet. In general, this has been accomplished through the use of coronagraphs. The design of coronagraphs can vary, including simple coronagraphs that utilize an occulting disk to block the star's light and nulling coronagraphs that use a phase mask to shift the phase of light, as opposed to directly blocking it. An occulting disk is usually used to block the sun and retrieve corona images. However, it is more suitable to use a phase mask for star systems because the entrance aperture dominates the diffraction effect of images.
In accordance with an aspect of the present invention, a phase mask assembly is provided. The phase mask assembly has a first surface and a second surface. The first surface and the second surface are configured such that the distance between the first surface and the second surface varies continuously in an azimuthal direction around the phase mask.
In accordance with another aspect of the present invention, a coronagraph apparatus is provided for separating a desired off-axis image from an undesired central image having a greater intensity. A first imaging lens receives light representing the desired off-axis image and the undesired central image. A reimaging portion mitigates the undesired central image. The reimaging portion includes a phase mask assembly that applies a continuous phase variation in an azimuthal direction to an image received from the first imaging lens, such that a null intensity region is produced in the center of the received image.
In accordance with yet another aspect of the present invention, a method is provided for producing a phase mask assembly that provides a continuous phase variation to incident light along an azimuthal direction. A desired wavelength is determined. An appropriate width variation is determined for the phase mask assembly according to the desired wavelength, such that the width of the phase mask varies continuously in an azimuthal direction. A transparent material is fabricated to exhibit the determined width variation.
The illustrated phase mask 10 is operative to change the phase of incident light of a given wavelength of interest continuously in an azimuthal direction. In accordance with an aspect of the present invention, the variation in thickness of the phase mask 10 from its thickest points 16 and 18, referred to as peaks, to its thinnest points 20 and 22, referred to as valleys, can be on the order of one wavelength. In the illustrated example, this variation of thickness is sinusoidal and can be described as
d(θ)=d0−a cos(2mθ) Eq. 1
where d0 is the average thickness, a is the variation amplitude such that the difference between a peak of the phase mask and a valley of the phase mask is equal to 2a, and m is the variation order. Such phase mask begins at a first peak 16 at a first azimuthal angle, drops to a first valley 20 at an angle ninety degrees from the first peak 16, rises to a second peak 18 at an angle 180 degrees from the first peak, and falls to a second valley 22 at an angle 270 degrees from the first peak before returning to the first peak to complete two full sinusoidal cycles. Since the thickness of the phase mask 10 varies azimuthally, light passing through various portions of the phase mask 10 will be slowed disproportionately, resulting in a continuous variation in phase in the azimuthal direction in light. The phase shifted light will have a null intensity at the center of the mask, but substantially no distortion away from the center of the phase mask, as the continuous phase shift ensures that there will be no phase discontinuity away from the center.
In the illustrated example, the phase profile 50 is sinusoidal to avoid phase singularities in the azimuthal direction away from the origin 56 of the phase profile. Specifically, the illustrated phase profile 50 can be described as:
φ(θ)=a cos(2nθ) Eq. 2
where φ(θ) is the phase at a point having an azimuthal coordinate θ, and n is the index of fraction of the material, and λ is the wavelength. When m is equal to one, the phase profile 150, and the corresponding width of the phase mask, will vary across two complete cycles in a single circumscription of the phase mask. It will be appreciated that more complex phase profiles can be generated using larger values of m, with the number of cycles associated with a circumscription of the phase mask increasing proportionately, such that four cycles are realized when m is equal to 2 and six cycles are realized when m is equal to three.
In order to have zero on-axis intensity, the phase profile 50 must satisfy the following relationship:
Accordingly, the variation amplitude, a, should be related to the zeros of a Bessel function of the zeroth order. If the refractive index n=1.5 and the first zero is chosen, the variation amplitude a can be found to be (a/λ)=2.405/π=0.766, which is about three-quarter of a wavelength.
A desired image, generally of a much lower intensity than the undesired central image, can be represented as a displaced secondary Airy pattern at the first focal plane 108. In order to suppress the primary Airy pattern at a reimaging plane 116, the phase mask 110 is placed at the first focal plane 108 to create, in concert with the second pupil 114 at the second lens plane 112, a null intensity on axis at the reimaging plane 116. The phase mask 110 can comprise a transparent material that is configured such that the phase of light passing through the phase mask is phase shifted azimuthally. For example, the thickness of the phase mask 110 can vary continuously in the azimuthal direction to apply the desired phase shift. As a result, the on-axis (e.g., central) intensity of the image, and accordingly, the majority of the primary Airy pattern, can be suppressed at the reimaging plane 116.
At 154, an appropriate width variation for the phase mask assembly is determined according to the desired wavelength. In accordance with an aspect of the present invention, the width of the phase mask will be fabricated to vary sinusoidally in an azimuthal direction. According, a phase shift applied to an image subjected to the phase mask such that the image will vary in phase continuously in an azimuthal direction. The amplitude of this sinusoidal variation can be determined, for example, as a zero associated with a zeroth order Bessel function. In accordance with an aspect of the present invention, this amplitude is close to three-quarters of the wavelength of interest, such that the difference in width between the widest portion of the phase mask and the narrowest portion of the phase mask will be approximately equal to one and one-half wavelength.
At 156, a transparent material is fabricated to the determined width variation. This can be accomplished, for example, via a diamond turning process using computer numerical controlled lathes to fabricate the transparent material into a shape having the determined width variation. At 158, the fabricated phase mask can be incorporated into a coronagraph intended for viewing an image having spectral properties associated with the wavelength of interest.
What has been described above includes exemplary implementations of the present invention. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the present invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims.
