BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a diffractive optical element and a method of producing the diffractive optical element.

Description of the Related Art

A phase modulation type diffractive optical element is used for branching a laser beam or changing the shape of a laser profile, and thus must have a form capable of providing different phase modulation for each region in the element to the incident light. In such a phase modulation type diffractive optical element, an engraving amount of the substrate is normally different depending on the position, for example, the engraving amount is large in a position where the phase modulation is large, and the engraving amount is small in a position where the phase modulation is small.

In order to continuously change the phase modulation amount of the phase modulation type diffractive optical element, the surface must be curved and the engraving amount of the element continuously change. However it is difficult to form a continuous curved surface, therefore the curved surface that is approximated to a discontinuous stepped form is often produced. It is relatively easy to produce this stepped form using a lithographic technique, which is a common semiconductor production process technique. But in order to improve diffraction efficiency, a number of steps of the step shape must be increased so that the accuracy of the approximation to an inclined plane is increased. If a number of steps increases, a number of times of the process increases too, which makes production difficult.

Japanese Patent Application Publication No. 2012-173555 discloses an optical element where a phase modulation function, equivalent to a step type diffraction grating, is provided by forming an uneven surface of which pitch is less than the wavelength of the incident light, and changing an effective refractive index, which is determined by a pitch or by a fill factor (ratio of width of a convex portion and a width of a concave portion) of the structure, for each location. With this method, however, it is necessary to form a structure of which fill factor is extremely large, or a structure of which fill factor is extremely small, to modulate the refractive index, and this makes stable production difficult.

In Japanese Patent Application Publication No. 2001-318217, an effective refractive index is modulated by adjusting the fill factor of the pillar or hole shapes, which are formed in a cycle that is not more than the wavelength of the incident light, and this structure is processed to have a predetermined depth, whereby a diffractive optical element having the phase modulation function is formed. In the case of this method, however, the fill factor of a structure that can be actually formed is limited, hence it is difficult to set a large modulation amount of the refractive index. If the modulation amount of the refractive index is small, the depth of the structure must be increased to implement a desired phase modulation amount, and this makes it difficult to form the structure.

SUMMARY OF THE INVENTION

With the foregoing in view, it is an object of the present invention to provide a phase modulation type diffractive optical element that can be easily produced, and a method of producing the diffractive optical element.

The first aspect of the present disclosure is a diffractive optical element having, on a surface of a transparent substrate, a plurality of types of regions which provide different phase modulation to an incident light, wherein each of the regions has a microstructure formed with concave and convex portions of which sizes are smaller than the wavelength of the incident light, and wherein in the microstructure, a ratio of the width of the convex portion and the width of the concave portion in the concave and convex portions and the depth of the concave portion are different for each type of region.

The second aspect of the present disclosure is a method of producing a diffractive optical element, comprising: a step of forming resist patterns having different cycles on a surface of a transparent substrate; and a step of forming a microstructure formed with concave and convex portions by performing dry etching using the resist patterns as a mask, wherein in the step of forming the microstructure, a microstructure in which depth of the concave portion is in accordance with the ratio of the width of the convex portion and the width of the concave portion in the concave and convex portions is formed by changing the etching rate in accordance with the interval of the resist patterns.

The third aspect of the present disclosure is a method of producing a diffractive optical element, comprising: a step of forming resist patterns having different cycles on a surface of a transparent substrate; and a step of forming a microstructure formed with concave and convex portions by performing dry etching using the resist patterns as a mask, wherein in the step of forming a microstructure, a microstructure in which depth of the concave portion is in accordance with the ratio of the width of the convex portion and the concave portion in the concave and convex portions is formed by performing dry etching under the etching conditions for etching the transparent substrate in tapered shapes.

According to the present invention, a diffractive optical element can be easily produced.

Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

