CCD resolution improvement using a fiber optic taper


Preface

With the development of photoelectric technology,the detector system based on CCD system have widely used in many field.Spatial resolution,is one of the major specifications of image system,it has strong relationship with optical system,CCD detector,Image Capture Card. And CCD detector use the pixel to describe the item,for the CCD camera with pixel around 10um,its ideal optical system cut-off frequency should be a 1 digit um on the image surface.In this case,CCD’s pixel size decide the camera’s resolution.However,it is very difficult and costly to make CCD with small pixel size, and this will finally limit the resolution in the imaging system.

Here,We introduce a way to improve CCD resolution by coupling CCD with Fiber Optic Taper. The Fiber optic taper is placed reversed on the CCD (Big end to CCD,and Small end to image).


Principles

a) Fiber Optic Taper’s Transmission Ratio

The light transmission performance is featured as transmission ratio.Transmission Ratio(T) defined as the input light(I) divided by output light(Io), T=I/Io,the major factors will affect fiber optic taper’s transmission ratio includes,the absorption of fiber;the total reflection between core and cladding;the Fresnel reflection and Fiber Optic Taper’s effective light transmission area.All these factors can make increase light loss. And the coupling part between FOT and CCD will also increase the light loss.

However,when incident light pass through fiber optic taper and optical epoxy layer.The coupling effciency equals to the beam reach to FOT compare with the total incident light power.This numer is related with N.A,and packing fraction of Fiber Optic Taper.This will finally result in great light loss after pass through the coupling Components.

b) The identify ability of Fiber Optic Taper

Fiber optic taper could be used for image transmission,in FOT each singe fiber will has a good optical insulation features.So that each fiber can transmit the light independently.Here we can take the input image as many many pixel with different brightness.Different end face of fiber optic taper could be seen as regularly arranged image dissector.Thus the pixel size will equal to the sample hole size.Pixel number will equal to the number of fiber filaments.So that each of the fiber will pass pixel through its own sampling hole,and two end of the FOT are arranged correlated.Due to the features described above,when an image is input on the FOT,it will be transfer through each fiber in the FOT. Then the image will be passed through the FOT.We usually use resolution for determining the image quality of FOT. And resolution are affected by Fiber’s Centering-Centering distance,Alignment,Scanning methods,and Fiber diameter. In the case,the fiber’s limiting resolution will be 1/2d(d=fiber’s diameter);for a hexagone components,it’s effective light transmission area equals to :

1

In the case of fibers are closely arranged,the effective area will account for 90.7% of total area.The limiting resolution will be :

2

The resolution is inversely to its fiber diameter,the smaller the fiber diameter,the higher the resolution.

When light come out from FOT’s small end to large end,it will have magnification to the image.Thus,we can use the big end of FOT to coupling with CCD for improving the image resolution.Following experiment will show the result.

(in the experiment we use the FOT with a hexagone shape fiber,and the magnification ratio is 2:1,when image pass through the FOT(reserved placed),the image is minified to 1/2 of the original image.But the resolution will multiple 2 times compares with no FOT is coupling.

c) The identify ability of CCD

Item could be imaging at a certain spatial frequency on CCD’s image plane. The resolution of CCD equal to the spatial frequency could be identified.In the experiment,according to the distance of CCD we could get image with different spatial frequency on the image plane of CCD. We call the maximum spatial frequency the “limit resolution of CCD”.Due to the magnification function of fiber optic taper,the reversed placed coupling components should be 2 times of CCD’s limit resolution.Vice versa.


*The relationship of limit resolution and FOT’s position

Normal Position Reversed Position
Fiber Optic Plate Fiber diameter(um) 6.3(big end) 4.0(small end)
Limit Resolution(lp/mm) 91.9 144.3
Pixel Size(um) 6.3 12.5
CCD Limit Resolution(lp/mm) 79.1 40.0
Coupling Components Limit Resolution(lp/mm) 39.6 80.0

Beampath of Experiment

In this experiment,we use a Black & white resolution test pattern. we use contrast in 2 condition for comparison. 1)Pass through optical system and Coupling components, 2) Pass through optical system and CCD components. The beampath are showed below.

Imaging System chart - RedrawIn this system focus distance=16mm, Pattern Period=2mm,4mm. For example in a 2mm  pattern plate,spatial frequency=0.5lp/mm,the spatial frequency on image plane will be 5lp/mm,assume object distance=u,image distance=v,we can conclude a chart below.


*Object Length and Image Length

Pattern Period 2mm 4mm
Spatial frequency(lp/mm) Image

Distance(mm)

Object Distance(mm) Image

Distance(mm)

Object Distance(mm)
5 17.60 176 16.80 336
10 16.80 336 16.40 656
15 16.53 496 16.27 976
20 16.40 656 16.20 1296
25 16.32 816 16.16 1616
30 16.30 976 16.13 1936
35 16.23 1136 16.11 2256
40 16.20 1296 16.10 2576
45 16.18 1456 16.09 2896
50 16.16 1616 16.08 3216

According to the equation below we can get :

Equation

Then, u=176 mm,v=17.6 mm


Thus,when we put the pattern plate in the distance of 176 mm to the lens, the Spatial frequency should be 5lp/mm,with the increase of Spatial frequency,the image will become blur and finally can’t be identified.

Following some photos for showing the experiment result.


 

4mm period 25lpmm non coupling

4mm Pattern 25lp/mm
-Non Coupled Device
4mm period 25lpmm coupling


4mm Pattern 25lp/mm
-Coupled Device
2mm period 25lpmm non coupling

2mm Pattern 25lp/mm
-Non Coupled Device
2mm period 25lpmm coupling

2mm Pattern 25lp/mm
-Coupled Device

And we can found,the image pass through coupling components will become dark,this is because the light loss during the transmission in coupling components.


Conclusion

  1. When observing the image,eyes are not able to identify the image below a contrast rate of 0.1,so we assume contrast ratio 0.1 is the limit frequency of according Spatial frequency.
  2.  Let’s take a look in the chart of reversed coupling components and non-coupling components’s  CTF contrast curve.The coupling components’ limit frequency is much more higher than the non-coupling one.
  3. For CTF contrast curve of the normal positioned coupling components and non-coupling components,we can found the normal positioned components’ limit frequency is lower than the non-coupling one.
  4. In the conclusion,we can use reversed placed FOT for improving the image resolution