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.