The present invention relates to the manufacture of optical systems and, more particularly, to a method for manufacturing a high-quality optical system for little more than the cost of a low-quality optical system.
The Hubble Space Telescope (HST) was launched into low Earth orbit in 1990 for the purpose of performing astronomical observations beyond the turbulence of the Earth's atmosphere. The main mirror of the HST was fabricated to within a precision of less than one-tenth of a wavelength of visible light, with the expectation that the images acquired by the HST would have a resolution near or at the mirror's diffraction limit.
It soon was discovered that the HST's main mirror had been fabricated very precisely to within less than one-tenth of a wavelength of visible light relative to the wrong shape. Therefore, corrective optics were fabricated, and installed in 1993, to correct the HST's optics.
Another approach to overcoming the influence of atmospheric turbulence on astronomical observations is to use adaptive optics to measure and compensate for the effect of the turbulence in real time. How this is done is illustrated schematically in
Iwasaki et al., in US Patent Application No. 2001/0028028 A1, teach a similar method for correcting aberration in an optical system that is used for reading optical disks.
Aberration correction optical unit 52 is a liquid crystal element sandwiched between two transparent electrodes. The alignment state of the liquid crystal element changes in response to the electric field between the electrodes that is induced by the applied voltage V. The shapes of the electrodes are selected according to the type of aberration (spherical or coma) to be corrected. The degree of correction is determined by the magnitude of the applied voltage V.
According to the present invention there is provided a method of making an optical system, including the steps of: (a) fabricating an optical subsystem of the optical system to within a first precision; (b) measuring an aberration of the optical subsystem to within a second precision that is more precise than the first precision; and (c) fabricating a static optical element that corrects the aberration to within the second precision.
According to the present invention there is provided an optical system including: (a) an optical subsystem fabricated to within a first precision; and (b) a static optical element, fabricated to within a second precision that is more precise than the first precision, for correcting an aberration of the optical subsystem.
According to the present invention there is provided a device for making an optical element, including: (a) a source for emitting a light wave that is a plane wave; (b) a spatial light modulator for modulating the plane wave in accordance with a predetermined profile, thereby transforming the plane wave into a modulated light wave; and (c) a projection system for projecting the modulated light wave onto a photosensitive workpiece, as one step in transforming the workpiece into the optical element.
According to the present invention there is provided a method of making an optical element that is configured with a predetermined profile, including the steps of: (a) modulating a light wave that is a plane wave in accordance with the profile, thereby transforming the plane wave into a modulated light wave; and (b) projecting the modulated light wave onto a photosensitive medium so as to prepare the photosensitive medium for transformation to the optical element.
According to the present invention there is provided a production line for making a plurality of optical systems for manipulating light to within a desired precision, including: (a) a first station for fabricating, for each optical system, a respective optical subsystem to within a preliminary precision that is less precise than the desired precision; (b) a second station for measuring a respective aberration of each optical subsystem to within the desired precision; and (c) a third station for fabricating, for each optical system, a respective static optical element that corrects the respective aberration to within the desired precision.
The present invention is a method for making a high-precision optical system at a cost that is little more than the cost of a comparable low-precision optical system. The cost of fabricating an optical component to high precision (one-tenth of a wavelength or better), as were the HST main mirror and the HST corrective optics, typically is one to two orders of magnitude greater than the cost of fabricating such an optical component to lower precision (e.g. one-half of a wavelength). For example, reflector telescope optics fabricated to a precision of one-tenth of a wavelength typically cost about $50,000. Comparable optics fabricated to a precision of one-half of a wavelength would cost about $2000. According to the present invention, an optical component that is a subsystem of a larger optical system is fabricated to relatively low precision. Then, the aberration of the optical component is measured to high precision, and a static optical element is fabricated to compensate for the measured aberration. Finally, the static optical element is fixed in place relative to the optical component so as to correct the aberration. The combination of the low-precision optical component and the static optical element has an optical performance comparable to the optical performance of a high-precision optical component, at a cost that is higher than the cost of the low-precision optical component alone but is only a fraction of the cost of the high-precision optical component.
The difference between the present invention and the HST is that in the case of the HST, both the main mirror and the corrective optics were fabricated to high precision, whereas according to the present invention, only the corrective optics, and not the optical subsystem whose aberration is being corrected, is fabricated to high precision. The difference between the present invention and the teachings of Iwasaki et al. is that the corrective optical element of the present invention is a static element, meaning that the optical properties of the element are fixed in advance and not changed during use, whereas aberration correction optical unit 52 of Iwasaki et al. is dynamic, in the sense that the degree of correction is changed during use by changing the applied voltage V.
Preferably, the measuring of the aberration of the optical subsystem is effected using interferometry. Alternatively, the measuring of the aberration of the optical subsystem is effected using a Shack-Hartman wavefront sensor.
Most preferably, the measuring of the aberration of the optical subsystem is effected as follows: A light wave that is initially a plane wave is passed through the optical subsystem and then is reflected by a deformable mirror that includes a plurality of actuators, each of which actuators positions a respective portion of the surface of the mirror. As is generally understood in the art, a “plane wave” is a coherent monochromatic light wave whose surfaces of constant phase are substantially flat and parallel. A property of the reflected light that is related to the aberration is measured, and the actuators are adjusted until the deformable mirror corrects the aberration to within the desired high precision to which the overall optical system is to be corrected. The static corrective optical element then is fabricated according to the final adjusted positions of the actuators.
Preferably, the property of the reflected light that is measured is the wavefront shape of the reflected light, and the actuators are adjusted until the measured wavefront shape is planar to within the desired high precision to which the overall optical system is to be corrected. Note that this preferred method of measuring the aberration differs from the use of a Shack-Hartman wavefront sensor to measure the aberration in that a Shack-Hartman wavefront sensor measures wavefront shape explicitly, whereas this preferred method of measuring the aberration measures wavefront shape only implicitly: the wavefront shape is sufficiently planar when the aberration is corrected to within the desired high precision.
Preferably, the adjustment of the actuators is effected using a nonlinear optimization algorithm, for example a simulated annealing algorithm or a genetic algorithm.
Optionally, the light that is reflected from the deformable mirror is passed again through the optical subsystem before its wavefront shape is measured. Preferably, the measuring of the wavefront shape is effected using a wavefront sensor.
Preferably, the static corrective optical element is fabricated by steps including configuring the shape of the static corrective optical element to correct the aberration of the optical subsystem. This shaping of the static corrective optical element is performed, for example, by photolithography or by laser ablation. Alternatively, the static corrective optical element is fabricated by steps including configuring the refractive index of the static corrective optical element to correct the aberration of the optical subsystem.
Preferably, the static corrective optical element is a transmissive optical element. Alternatively, the static corrective optical element is a reflective optical element.
The scope of the present invention also includes a device and method for making an optical element, such as the static optical element, in accordance with a predetermined profile.
The device for making the optical element includes a source that emits a light wave that is a plane wave, a spatial light modulator for modulating the plane wave in accordance with the profile, and a projection system for projecting the modulated light wave onto a photosensitive workpiece. Preferably, the spatial light modulator is a liquid crystal spatial light modulator. Preferably, the projection system includes a first lens, a second lens, and an aperture, between the two lenses, for allowing only the first order diffraction pattern of the modulated light wave from the first lens to reach the second lens. Most preferably, the lenses are Fourier transform lenses. Preferably, the photosensitive workpiece includes photoresist.
The method of making the optical element includes the steps of modulating a light wave that starts out as a plane wave in accordance with the profile, projecting the modulated light wave onto the workpiece, and developing the workpiece for an amount of time sufficient to configure the workpiece with the desired profile.
The scope of the present invention also includes a production line for making a plurality of optical systems using the method of the present invention. The production line includes four stations. At the first station, an optical subsystem of each system is fabricated to low precision. At the second station, the aberration of each optical subsystem is measured to high precision. At the third station, static optical elements are fabricated for correcting the aberrations to high precision. At the fourth station, each static optical element is mated to its respective optical subsystem and fixed in place so that the combination of the optical subsystem and the static optical element constitute a high-precision optical system, with the aberration of the optical subsystem being corrected to high precision by the static optical element.
The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:
The present invention is of a method of manufacturing optical systems. Specifically, the present invention can be used to manufacture a high-quality optical system for little more than the cost of a comparable low-quality optical system.
The principles and operation of optical system manufacture according to the present invention may be better understood with reference to the drawings and the accompanying description.
Returning now to the drawings,
Static corrective optical element 64 is represented in
In what follows, the direction of light propagation is assumed to be parallel to the z-axis of a Cartesian (x,y,z) coordinate system. Quantitatively, the aberration represented by wavefront 66 is a phase deviation Δφ(x,y) from the design phase of coherent monochromatic light of wavelength λ. To design static corrective optical element 64, this phase deviation must be measured. Methods of measuring this phase deviation are well-known in the prior art. Among these methods are interferometry, as described in Optical Shop Testing (Daniel Malacara, editor) (John Wiley & Sons, 1978) (see especially page 76); and the use of a Shack-Hartman wavefront sensor, as described in the Malacara book and also by W. H. Southwell in “Wavefront estimation from wave-front slope measurements”, J. Opt. Soc. Am. vol. 70 pp. 998-1006 (1980).
Another method of measuring the phase deviation is illustrated schematically in
One kind of static corrective optical element 64 that corrects for a given phase deviation Δφ(x,y) is a thin transparent plate, with an index of refraction n, flat to within the desired precision on one side and with the other side contoured with a profile
Such a static corrective optical element 64 is a transmissive optical element Another kind of static corrective optical element 64 that corrects for a given phase deviation Δφ(x,y) is a reflective optical element: a mirror with a profile PR(x,y)=0.25λΔφ(x,y)/π.
with pixels at coordinates (x,y) such that
being totally opaque. A plane wave 93 from a source 92 is modulated by SLM 94 and then is projected onto a rigid sheet 102 of positive photoresist by a projection system that includes two Fourier transform lenses 96 and 98 separated by an aperture 100. Aperture 100 acts as a spatial filter to allow only the first order diffraction pattern of the modulated light from lens 96 to reach lens 98, in order to reduce the pixellation of the light that reaches sheet 102. Note that the wavelength of plane wave 93 generally is not the same as the wavelength of plane wave 70: plane wave 93 is supposed to induce a chemical change in sheet 102, whereas the optical element that sheet 102 eventually becomes is supposed to be insensitive to plane wave 70. For example, plane wave 70 may be visible or infrared light, and plane wave 93 may be ultraviolet light. After sheet 102 is exposed in this manner, sheet 102 is immersed in a developer. The depth to which material is dissolved by the developer from the surface of sheet 102 that was exposed to the light, as a function of lateral coordinates (x,y), is proportional to both the integrated flux of light to which sheet 102 was exposed at coordinates (x,y) and the amount of time that sheet 102 remains in the developer. The total development time is selected so that the final shape of the exposed side of sheet 102 is PT(x,y).
The discussion above assumes that the depth to which the light from source 92 modifies the chemistry of the photoresist of sheet 102 is a linear function of the cumulative intensity of the light impinging on the photoresist. In some photoresists, this function is nonlinear. When such photoresists are used, the opacities of the pixels of SLM 94 are modified accordingly.
Alternatively, sheet 102 is made of a photoresist whose index of refraction is modified by exposure to the light from source 92. The required change in the index of refraction n is
where L is the thickness of sheet 102.
As another alternative, static corrective optical element 64 is made by laser ablation of a rigid transparent sheet.
While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.
Number | Date | Country | Kind |
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157920 | Sep 2003 | IL | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IL04/00829 | 9/9/2004 | WO | 5/3/2007 |