# US 11,079,304 B2 · Focal-length measuring apparatus for sub wavelength optical imaging device and measuring method thereof

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- Number: US 11,079,304 B2
- Document ID: US11079304B2
- Status: Granted
- Kind: B2
- Application number: 16/468,283
- Filed: 2018-10-18
- Published: 2021-02-25
- Granted: 2021-08-03
- Earliest claimed: 2018-09-13
- Assignees: Xihua University
- Applicants: Xihua University
- Inventors: Wenbo Jiang; Yun Bu; Nan Wang; Xiao Ren
- Web page: https://patentgrep.com/patent/US11079304B2

## CPC

- G01M 11/0228: {by measuring refractive power}
- G01M 11/0264: {by using targets or reference patterns}
- G02B 27/30: Collimators
- G02B 27/48: Laser speckle optics
- G01M: TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
- G02B: OPTICAL ELEMENTS, SYSTEMS OR APPARATUS

## Abstract

A focal-length measuring apparatus for a sub-wavelength optical imaging device includes a laser, a beam-expanding and collimating system, a sub-wavelength optical imaging device, and a nanoscale stepped height standard sample block. The nanoscale stepped height standard sample block is connected to a power device, and the power device is connected to a computer control system. The nanoscale stepped height standard sample block is coated with a photoresist and includes a plurality of steps arranged at equal intervals Among all the steps, the heights of the steps gradually increase from a middle step to an upper side, and the values of the corresponding focal lengths decrease. While, the heights of the steps gradually decrease from the middle step to a lower side, and values of corresponding focal lengths increase. A wavelength of the laser is equal to a designed wavelength of an input light source of the sub-wavelength optical imaging device.

## Claims

### Claim 1

A focal-length measuring apparatus for a sub-wavelength optical imaging device comprising a laser, a beam-expanding and collimating system configured to turn light emitted by the laser into even and collimated parallel light, a sub-wavelength optical imaging device configured to focus the parallel light on a focal plane and form speckles, and a nanoscale stepped height standard sample block placed on the focal plane;
wherein, the nanoscale stepped height standard sample block is connected to a power device for driving the nanoscale stepped height standard sample block to move up and down in a vertical direction, and the power device is connected to a computer control system; and
wherein, a surface of the nanoscale stepped height standard sample block is coated with a photoresist and the nanoscale stepped height standard sample block comprises a plurality of steps arranged at equal intervals on a same vertical plane, one of the steps is a middle step, a distance between the middle step and the sub-wavelength optical imaging device is a designed focal length of the sub-wavelength optical imaging device; among all the steps, heights of the steps gradually increase from the middle step to an upper side, and values of corresponding focal lengths decrease; heights of the steps gradually decrease from the middle step to a lower side, and values of corresponding focal lengths increase; and a wavelength of the laser is equal to a designed wavelength of an input light source of the sub-wavelength optical imaging device, wherein the sub-wavelength optical imaging device is a micro/nano optical lens, wherein the focal-length apparatus has no piezoelectric ceramic to control a movement of the focal-length apparatus,
wherein when there is an error ±ΔXN between an actual focal length and the designed focal length, the measuring method comprises the following steps:
adjusting the sub-wavelength optical imaging device till its focus is focused on the middle step of the nanoscale stepped height standard sample block, wherein at this time, a corresponding focal length is fo;
driving the nanoscale stepped height standard sample block to move upward a predetermined distance by the power device controlled by the computer control system, focusing the focus of the sub-wavelength optical imaging device on an above step in a forward direction, and accomplishing a speckle exposure process in the forward direction for one time;
wherein, the predetermined distance is equal to an interval between two adjacent steps;
returning back to perform the previous step till the focus of the sub-wavelength optical imaging device moves onto an Nth step, and accomplishing the speckle exposure process in the forward direction for an Nth time, wherein a corresponding focal length after moving by the predetermined distance each time is fo−Δxn, 1<n<N;
moving the power device in a counter direction by the computer control system till the focus of the optical imaging device is focused on the middle step of the nanoscale stepped height standard sample block;
driving the nanoscale stepped height standard sample block to move downward a predetermined distance by the power device controlled by the computer control system, focusing the focus of the sub-wavelength optical imaging device on a next step in the counter direction and accomplishing a speckle exposure in the counter direction for one time:
returning back to perform the previous step till the focus of the sub-wavelength optical imaging device moves onto an Nth step in the counter direction, and accomplishing the speckle exposure process in the counter direction for an Nth time, wherein a corresponding focal length after moving by the predetermined distance each time is fo+Δxn;
developing an exposed nanoscale stepped height standard sample block by using a developing solution to obtain a series of unequal-sized speckles after a focusing of the sub-wavelength optical imaging device; and
observing by an atomic force microscope and measuring a size of each of the speckles, wherein if the speckle of one step has a minimum size, a corresponding focal length of the step is the actual focal length of the sub-wavelength optical imaging device.

### Claim 2 (depends on 1)

The focal-length measuring apparatus for a sub-wavelength optical imaging device according to claim 1, wherein height differences between adjacent steps are not equal; closing to the middle step, the height differences between adjacent steps are decreased; departing from the middle step, the height differences between adjacent steps are increased.

### Claim 3 (depends on 1)

The focal-length measuring apparatus for a sub-wavelength optical imaging device according to claim 1, wherein the height differences between adjacent steps are equal.

### Claim 4 (depends on 1)

The focal-length measuring apparatus for a sub-wavelength optical imaging device according to claim 1, wherein the power device is a stepping motor.

### Claim 5 (depends on 1)

The focal-length measuring apparatus for a sub-wavelength optical imaging device according to claim 1, wherein the intervals between adjacent steps range from 50 μm-100 μm.

### Claim 6 (depends on 1)

The focal-length measuring method according to claim 1, wherein
if the height differences between adjacent steps are all equal to each other, namely, ΔxN−ΔxN-1=ΔxN-1−ΔxN-2= . . . =Δx2−Δx1, and Δxi−Δxi-1=C is recorded, 1<i≤N; wherein C is a constant value;
if the height differences between adjacent steps are not equal to each other, namely, ΔxN−ΔxN-1≠ΔxN-1−Δxn-2≠ . . . ≠Δx2−Δx1, and Δxi−Δxi-1=Bi is recorded, wherein Bi is a variable, a decrease of i indicates getting closer to the middle step and a decrease of Bi; and an increase of i indicates getting far away from the middle step and an increase of Bi.

### Claim 7 (depends on 2)

The focal-length measuring apparatus for a sub-wavelength optical imaging device according to claim 2, wherein the intervals between adjacent steps range from 50 μm-100 μm.

### Claim 8 (depends on 3)

The focal-length measuring apparatus for a sub-wavelength optical imaging device according to claim 3, wherein the intervals between adjacent steps range from 50 μm-100 μm.

### Claim 9 (depends on 4)

The focal-length measuring apparatus for a sub-wavelength optical imaging device according to claim 4, wherein the intervals between adjacent steps range from 50 μm-100 μm.

### Claim 10 (depends on 1)

The focal-length measuring method according to claim 1, wherein height differences between adjacent steps are not equal; closing to the middle step, the height differences between adjacent steps are decreased; departing from the middle step, the height differences between adjacent steps are increased.

### Claim 11 (depends on 1)

The focal-length measuring method according to claim 1, wherein the height differences between adjacent steps are equal.

### Claim 12 (depends on 1)

The focal-length measuring method according to claim 1, wherein the power device is a stepping motor.

### Claim 13 (depends on 1)

The focal-length measuring method according to claim 1, wherein the intervals between adjacent steps range from 50 μm-100 μm.