Optical: systems and elements – Diffraction – From grating
Reexamination Certificate
1998-05-11
2001-09-18
Chang, Audrey (Department: 2872)
Optical: systems and elements
Diffraction
From grating
C359S569000, C359S566000, C359S571000, C359S573000
Reexamination Certificate
active
06292297
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a multilevel diffractive optical element (DOE), in particular to a computer generated phase DOE, comprising a substrate with a substantially periodic transmissive or reflective relief pattern of phase retardation zones.
BACKGROUND OF THE INVENTION
Computer generated DOEs of the above kind are capable of performing complicated phase transformations of a radiation wave incident thereon such as a conversion of incident radiation wavefront having one shape into a wavefront of any other shape. DOEs of the specified kind are usually designed to have a high diffraction efficiency at a predetermined, most often, first diffraction order.
In order to obtain 100% diffraction efficiencies, DOEs suggested by Jordan et al and known as kinoforms have a periodic blazed surface relief with phase zones having a continuous profile (“Kinoform lenses”, Appl. Opt., 9/8, August 1970, pp. 1883-1887). The depth of the phase zones in kinoforms is generally proportional to phase residues after modulo 2&pgr; so that, in each phase zone, phase variations range is from 0 to 2&pgr;. However, it is practically very difficult to produce high quality kinoforms with properly shaped continuous blazed profile.
It has, therefore, been suggested to quantize the ideal continuous phase profile of the DOEs into discrete phase levels as an approximation to the continuous profile. Manufacturing of such a multilevel DOE is based on a generation of a plurality of binary amplitude masks and their serial use for serial etching of a plurality of levels over the entire optical element. Thus, for example, a multilevel DOE disclosed in U.S. Pat. No. 4.895,790, is produced by means of M masks in M serial manufacturing cycles so that, at each manufacturing cycle, each previously produced level is divided into two levels with a smaller distance therebetween. Thereby, in each phase zone of the DOE, there are produced N=2
M
levels spaced by identical distances having and boundaries defining equiphase areas of the DOE. However, due to the fact that in such a multilevel DOE, all the phase zones have identical depth and number of phase levels, an amplitude of the diffracted wavefront cannot be changed independently of its phase and therefore, a desired distribution of overall diffraction efficiency of such a DOE cannot be achieved. Furthermore, when a multilevel DOE of the above kind has a varying grating period, such as for example in case of high numerical aperture diffractive lenses, maximal local diffraction efficiencies cannot be simultaneously obtained from all the phase zone, whereby overall diffraction efficiency of the DOE is reduced.
To provide for an independent control of an amplitude of diffracted wavefront, in a binary DOE, Brown, B. R. and Lohmann, A, W. have suggested a DOE in which the amplitude of the diffracted wavefront is controlled by an appropriate choice of the ratio between the widths of the levels (Brown, B. R. and Lohmann, A, W., “Complex spatial filtering of binary masks”, Applied Optics, 5 June 1966, p. 967). However, with the number of phase levels being limited to two, the diffraction efficiency of the DOE cannot exceed 40.5%.
It is the object of the present invention to provide a new computer generated multilevel phase diffractive optical element, in which local diffraction efficiencies and consequently an overall diffraction efficiency can be arbitrarily controlled in the range from 0 to nearly 100% over the entire element.
SUMMARY OF THE INVENTION
In the following description and claims the term “profile” used with respect to a multilevel phase zone of a diffractive optical element means a line passing through extremities of phase levels of the phase zone. The term “modulation depth” of a multilevel phase zone means a distance from the uppermost level of the phase zone to a base of the diffractive optical element. The term “optimal modulation depth” with respect to a multilevel phase zone means a modulation depth proportional to phase residues after modulo 2&pgr;, which the phase zone would have, in order to ensure 100% diffraction efficiency in an m-th diffraction order, if the phase zone were continuous rather than multilevel. When a multilevel phase zone has such an optimal modulation depth, an angle of inclination of its profile with respect to the base of the diffractive optical element is optimal and a diffraction efficiency provided thereby is nearly 100% The term “local” with respect to any feature of a diffractive optical element is used to designate a magnitude or value which this feature has at one specific location of the diffractive element. Thus, for example, a local modulation depth of a phase zone is a modulation depth seen in a cross-sectional view of the phase zone taken at one location along the extension thereof.
In accordance with the present invention there is provided a multilevel diffractive optical element comprising a base and a plurality of phase zones defined by a modulation depth and a number of phase levels, the number of the phase levels per phase zone varying at different locations of the element, characterised in that the variation of said number of phase levels is such that the modulation depth, at said different locations, varies in a predetermined manner of the element.
Thus, by the appropriate choice of local modulation depth, according to the present invention, it is ensured that at each location of the diffractive optical element, the phase zone profile is inclined with respect to the base of the element in such a manner that a local amplitude of the diffracted wavefront and, consequently, a local diffraction efficiency obtained from the diffractive optical element, at each said location thereof, have predetermined values.
The required orientation of the phase zone profile may be achieved by pivoting of a profile which forms with the base of the DOE an optimal angle around its central point or one of its edge points or any other, arbitrarily chosen point.
Thus, by virtue of variation of the modulation depth over the entire element, e.g. from phase zone to phase zone and/or within one phase zone along the direction of the extension thereof, any required distribution of diffraction efficiency of the element can be achieved. Particularly, it can be provided that, at any location of the DOE, a local diffraction efficiency in the desired order is nearly 100%. This will happen in case when, at said location of the element, the local modulation depth is of its optimal magnitude.
The local modulation depth at each location of the element is defined by the local number of phase levels at this location and by the distance therebetween. Due to the fact that, in practice, it is extremely complicated to form DOEs having variable distance between levels, in the DOE according to the present invention the distance between of phase levels is preferably invariant over the entire element.
In order to determine a specific magnitude of the distance between phase levels it should be kept in mind that the smaller the distance between phase levels height, the greater the number of phase levels which is required for the provision of a desired modulation depth and that, in order to render the manufacturing of the DOE less complicated and to reduce fabrication errors and scatter noise, it is clearly desirable to minimize the number of phase levels and, consequently to choose a maximal possible distance therebetween. On the other hand, to obtain required diffraction efficiencies, the number of phase levels should not be unduly minimized and therefore, the distance between levels must be sufficiently small, being however not less than that dictated by manufacturing constrains.
In view of the above, it is suggested, according to the present invention, that the distance between phase levels has an optimized magnitude determined as a distance between phase levels of a phase zone in which d
opt
/N
min
is of a minimal value, where d
opt
is a local optimal modulation depth of the phase zone and N
min
is a minimal local number of le
Danziger Yochay
Friesem Asher
Hasman Erez
Chang Audrey
Nath Gary M.
Nath & Associates PLLC
Novick Harold L.
Yeda Research and Development Co. Ltd.
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