Phase-transforming optical reflector formed by partial etching or by partial etching with reflow

• US 10,539,723 B2
• Filed: 10/16/2017
• Issued: 01/21/2020
• Est. Priority Date: 10/19/2016
• Status: Active Grant

• Current Assignee: II-VI Delaware, Inc. (Coherent Corp.)
• Original Assignee: Finisar Corporation (II-VI Incorporated)
• Inventors: Iazikov, Dmitri, Mossberg, Thomas W., Greiner, Christoph M., Clark, John H.
• Primary Examiner(s): Shafer, Ricky D
• Application Number:
  US15/784,702
• Publication Number:
  US 20180128948A1
• Time in Patent Office: 827 Days
• Field of Search: 359565, 359572, 359851, 359900
• CPC Class Codes: C23F 1/02 Local etching
• CPC Class Codes: G02B 3/08 with discontinuous faces, e.g. Fresnel lens diffractive Fresnel lenses G02B5/1876
• CPC Class Codes: G02B 5/0883 with a refractive index gradient rugate filters G02B5/289
• CPC Class Codes: G02B 5/1847 Manufacturing methods
• CPC Class Codes: G02B 5/1861 Reflection gratings characterised by their structure, e.g. step profile, contours of substrate or grooves, pitch variations, materials G02B5/1809, G02B5/1828, G02B5/1833, G02B5/1838 and G02B5/1847 take precedence
• CPC Class Codes: G02B 5/1866 Transmission gratings characterised by their structure, e.g. step profile, contours of substrate or grooves, pitch variations, materials G02B5/1809, G02B5/1828, G02B5/1833, G02B5/1838 and G02B5/1847 take precedence
• CPC Class Codes: G02B 5/1871 Transmissive phase gratings
• CPC Class Codes: G02B 5/3083 Birefringent or phase retarding elements G02B5/3008, G02B5/3016 take precedence; systems for polarisation control G02B27/286; manufacturing phase modulating patterns by lithographic processes G03F7/001
• CPC Class Codes: H01J 37/3056 for microworking, e.g. etching of gratings, trimming of electrical components trimming of resistors H01C17/22

Abstract

A reflective optical element includes a reflective surface comprising a multitude of discrete recessed and non-recessed areas arranged along the reflective surface. The discrete areas are arranged to approximate a desired phase function (typically modulo 2π) and are smaller than an operational wavelength in order to provide a range of phase delays needed to adequately approximate the desired phase function. Effecting at least partial reflow of one or more optical media or reflective materials can smooth the morphology of the reflective surface so as to reduce unwanted diffraction or scattering.

Claims

1. An optical element comprising a reflective surface supported by a substantially solid substrate, the reflective surface comprising a multitude of discrete recessed and non-recessed areas arranged contiguously along the reflective surface, wherein:

(a) the recessed or non-recessed areas include a non-empty subset of areas of the multitude having a largest transverse dimension less than about a design vacuum wavelength λ

₀, and the recessed and non-recessed areas are characterized by a depth function d(x,y), as a function of two-dimensional position coordinates x and y along the reflective surface, that equals zero in the non-recessed areas and that assumes non-zero values in the recessed areas up to a maximum depth D_R;

(b) the reflective surface is reflective over an operational wavelength range that includes the design vacuum wavelength λ

₀, and a corresponding volume within each recessed area is substantially transparent over the operational wavelength range and is characterized by a first bulk refractive index n₁(λ

);

(c) the optical element is structurally arranged so as to receive an optical signal incident on the reflective surface and to reflect at least a portion of the incident optical signal transformed substantially according to a specified effective phase transformation function φ

_eff(x,y) that varies as a function of x and y; and

(d) the discrete areas of the multitude are variously sized and distributed on the reflective surface so as to impart on the reflected portion of the incident optical signal the effective phase transformation φ

_eff(x,y).

2. The optical element of claim 1 wherein the discrete areas of the multitude are sized and distributed along the reflective surface so that (4π

/λ

₀)·

n₁(λ

₀)·

d(x,y), as a function of two-dimensional position coordinates x and y along the reflective surface, averaged over a sampling area having transverse dimensions about equal to λ

₀ along the reflective surface, is substantially proportional to or substantially equal to the effective phase transformation function φ

_eff(x,y).

3. The optical element of claim 1 wherein the optical element is structurally arranged so as to receive the incident optical signal at substantially normal incidence.

4. The optical element of claim 1 wherein the optical element is structurally arranged so as to receive the incident optical signal at non-normal incidence.

5. The optical element of claim 1 wherein the depth function d(x,y) equals zero in the non-recessed areas and is substantially equal to D_R in the recessed areas.

6. The optical element of claim 1 wherein the corresponding volume within each recessed area comprises a substantially solid material characterized by the bulk refractive index n₁(λ

).

7. The optical element of claim 6 wherein the substantially solid material characterized by the bulk refractive index n₁(λ

) forms a layer over areas of the reflective surface corresponding to the non-recessed areas.

8. The optical element of claim 1 wherein the corresponding volume within each recessed area (i) comprises vacuum characterized by the bulk refractive index n₁(λ

)=1, or (ii) comprises a gaseous or liquid material characterized by the bulk refractive index n₁(λ

).

9. The optical element of claim 1 further comprising a layer of reflective material supported by the substrate, wherein the reflective surface comprises recessed areas and non-recessed areas formed on a surface of the reflective material layer.

10. The optical element of claim 9 wherein the reflective material is a metallic material.

11. The optical element of claim 1 further comprising (i) a material layer supported by the substrate and (ii) a reflective film or layer, wherein the reflective surface comprises recessed areas and non-recessed areas formed on a surface of the material layer and covered with the reflective film or layer.

12. The optical element of claim 11 wherein each recessed area of the material layer forms a corresponding non-recessed area of the reflective surface, each recessed area of the material layer forms a corresponding non-recessed area of the reflective surface, the material layer is substantially transparent over the operational wavelength range and characterized by the bulk refractive index n₁(λ

), and the optical element is structurally arranged so as to receive the optical signal incident on the reflective surface by transmission of the optical signal through the material layer.

13. The optical element of claim 11 wherein each recessed area of the material layer forms a corresponding recessed area of the reflective surface, each non-recessed area of the material layer forms a corresponding non-recessed area of the reflective surface, and the optical element is structurally arranged so as to receive the optical signal incident on the reflective surface without transmission of the optical signal through the material layer.

14. The optical element of claim 1 wherein the depth D_R is substantially equal to [N/λ

₀]/[2n₁(λ

₀)], where N is a non-zero integer.

15. The optical element of claim 1 wherein φ

_eff(x,y) varies with both x and y.

16. The optical element of claim 1 wherein the position-dependent effective phase transformation function φ

_eff(x,y) is a modulo 2π

function.

17. The optical element of claim 1 wherein the effective phase transformation function φ

_eff(x,y) approximates a function of the form φ

(x,y)=Ax²+By², or φ

(x,y)=Ax²+By² modulo 2π

, where A and B are non-zero, positive or negative real numbers.

18. The optical element of claim 1 wherein the effective phase transformation function φ

_eff(x,y) approximates a function of the form φ

(θ

)=Mθ

, or φ

(θ

)=Mθ

modulo 2π

, for 0≤

θ

<

2π

, where θ

is related to x and y by cos θ

=x/(x²+y²)^1/2 and sin θ

=y/(x²+y²)^1/2 and M is a non-zero integer.

19. The optical element of claim 1 wherein the effective phase transformation function φ

_eff(x,y) approximates a sum, or a modulo 2π

sum, of distinct, specified, position-dependent phase transformation functions φ

₁(x,y) and φ

₂(x,y).

20. The optical element of claim 19 wherein (i) φ

₁(x,y)=Ax²+By², or φ

₁(x,y)=Ax²+By² modulo 2π

, where A and B are non-zero, positive or negative real numbers, and (ii) φ

₂(x,y)=Mθ

, or φ

(θ

)=Mθ

modulo 2π

, for 0≤

θ

<

2π

, where θ

is related to x and y by cos θ

=x/(x²+y²)^1/2 and sin θ

=y/(x²+y²)^1/2 and M is a non-zero integer.

21. The optical element of claim 1 wherein each discrete area of the multitude has a smallest transverse dimension no smaller than about λ

₀/K, where 2≤

K≤

20.

22. The optical element of claim 1 wherein the discrete areas are spatially distributed along the reflective surface in an uncorrelated, irregular, random, or pseudo-random arrangement.

23. The optical element of claim 1 wherein the discrete areas of the multitude are arranged according to a regular two-dimensional grid pattern along the reflective surface characterized by a grid spacing of Λ

₀ between about λ

₀/20 and about λ

₀.

24. The optical element of claim 23 wherein the multitude of discrete areas is arranged so that, within each unit cell of the grid pattern, the discrete areas or portions thereof encompassed by that unit cell are arranged according to one of a set of K predetermined unit arrangements, so that (4π

/λ

₀)·

n₁(λ

₀)·

d(x,y), averaged over each unit cell of the grid pattern, can assume one of K discrete values.

25. The optical element of claim 23 wherein the multitude of discrete areas is arranged so that (i) the discrete areas or portions thereof encompassed by each unit cell of the grid pattern are arranged as a single simply connected recessed area surrounded by a non-recessed area or (ii) the discrete areas or portions thereof encompassed by each unit cell of the grid pattern are arranged as a single simply connected non-recessed area surrounded by a recessed area, so that (4π

/λ

₀)·

n₁(λ

₀)·

d(x,y), averaged over each unit cell of the grid pattern, can assume a substantially continuous range of values according to a size of each unit cell occupied by the corresponding simply connected area.

26. A method employing the optical element of claim 1, the method comprising (i) directing the optical signal onto the reflective surface of the optical element and (ii) reflecting from the optical element at least a portion of the incident optical signal transformed substantially according to the effective phase transformation function φ

_eff(x,y).

27. A method for making the optical element of claim 1, the method comprising spatially selectively processing a surface of a material layer to form the multitude of recessed and non-recessed areas of the reflective surface.

28. The method of claim 27 further comprising filling the recessed areas of the reflective surface with a transmissive material that is substantially transparent over the operational wavelength range and characterized by the bulk refractive index n₁(λ

).

29. The method of claim 28 wherein the transmissive material is a substantially solid material and forms a layer over the non-recessed areas of the reflective surface.

30. The method of claim 29 wherein the material layer comprises a reflective material.

31. The method of claim 29 further comprising forming, depositing, or applying a reflective film or layer onto the recessed and non-recessed areas of the material layer to form the reflective surface.

32. A method for making the optical element of claim 1, the method comprising:

(A) specifying the effective phase transformation function φ

_eff(x,y);

(B) forming an intermediate optical element by spatially selectively processing a surface of a material layer to form a multitude of recessed and non-recessed areas of a reflective surface of the intermediate optical element in accordance with an estimated arrangement of the areas of the multitude;

(C) measuring an intermediate phase function φ

_inter(X,y) imparted on a portion of an incident optical signal reflected by the intermediate optical element;

(D) altering the estimated arrangement of the areas of the multitude in accordance with a difference between φ

_eff(x,y) and φ

_inter(x,y); and

(E) repeating steps (B), (C), and (D) using successively altered arrangements of the areas of the multitude until the difference between φ

_eff(x,y) and φ

_inter(x,y) is less than a specified allowable phase error,

(F) wherein the optical element is the intermediate optical element having the difference between φ

_eff(x,y) and φ

_inter(x,y) that is less than the specified allowable phase error.

33. A method for making the optical element of claim 1, the method comprising:

(A) spatially selectively processing a layer of a first material to form a multitude of discrete recessed and non-recessed areas arranged contiguously along the reflective surface according to an initial depth function d_I(x,y) that is a function of two-dimensional position coordinates x and y along the reflective surface; and

(B) after part (A), heating the spatially processed first material layer so as to cause at least partial reflow of the first material, so that the reflective surface is arranged according to a non-negative reflow depth function d_R(x,y) that is equal to d(x,y),

wherein;

(C) d(x,y)≠

d_I(x,y) over at least one or more portions of the reflective surface; and

(D) the discrete areas of the multitude are variously sized and distributed on the reflective surface so that, after parts (A) and (B), the reflective surface is arranged so as to impart on the reflected portion of the incident optical signal the effective phase transformation φ

_eff(x,y).

34. The method of claim 33, the method further comprising:

(A′

) specifying the effective phase transformation function φ

_eff(x,y);

(B′

) performing the method of claim 33 to form an intermediate optical element in accordance with an estimated arrangement of the areas of the multitude and an estimated reflow process sequence;

(C′

) measuring an intermediate phase function φ

_inter(x,y) imparted on a portion of an incident optical signal reflected by the intermediate optical element;

(D′

) altering the estimated arrangement of the areas of the multitude, or the reflow process sequence, in accordance with a difference between φ

_eff(x,y) and φ

_inter(X,y); and

(E′

) repeating steps (B′

), (C′

), and (D′

) using successively altered arrangements of the areas of the multitude or reflow process sequences until the difference between φ

_eff(x,y) and φ

_inter(x,y) is less than a specified allowable phase error,

(F′

) wherein the optical element is the intermediate optical element having the difference between φ

_eff(x,y) and φ

_inter(x,y) that is less than the specified allowable phase error.

35. The method of claim 33 wherein, after part (A) and before part (B), the discrete areas of the multitude are distributed along the reflective surface so that (4π

/λ

₀)·

n₁(λ

₀)·

d_I(x,y), averaged over a sampling area having transverse dimensions about equal to λ

₀ along the reflective surface, is substantially proportional to or substantially equal to φ

_eff(x,y), where (i) d_I(x,y) equals a minimum depth D_NR in the non-recessed areas, and (ii) d_I(x,y) assumes non-zero values up to a maximum depth D_R in the recessed areas.

36. The method of claim 33 further comprising, after part (A) and before part (B), at least partly filling the recessed areas with a second material that is substantially transparent over the operational wavelength range and characterized by the bulk refractive index n₁(λ

), wherein the heating of part (B) causes at least partial reflow of the second material.

37. The method of claim 33 further comprising, after parts (A) and (B), at least partly filling the corresponding volumes, located above at least portions of the reflective surface for which d_R(x,y)>

0, with a second material that is substantially transparent over the operational wavelength range and characterized by the bulk refractive index n₁(λ

).

38. The method of claim 33 further comprising, after parts (A) and (B), forming a reflective film or coating on the reflective surface.

39. The method of claim 33 further comprising, after part (A), forming a layer of reflective material over the first material layer, wherein (i) part (A) comprises spatially selectively etching the first material layer, (ii) resulting etched areas of the first material form the non-recessed areas;

(iii) the substrate and the first material are substantially transparent over the operational wavelength range, (iv) the first material is characterized by the bulk refractive index n₁(λ

) and fills the corresponding volumes located above at least portions of the reflective surface for which d_R(x,y)>

0, and (v) the optical element is arranged so that the optical signal incident on the reflective layer propagates through the substrate.

40. The method of claim 39 wherein the layer of reflective material is formed over the first material layer before part (B), and the heating of part (B) causes at least partial reflow of the reflective material.

41. The method of claim 39 wherein the layer of reflective material is formed over the first material layer after part (B).