Important Metrics in Fiber Imaging Component
The preparation process of optical fiber imaging components is complex and can be divided into six stages. Each stage is subdivided into processes. Each preparation process may affect its performance. Each performance is a one-vote veto on whether the product is qualified or not.
Among many performances, imaging performance is the core. Of course, since optical fiber imaging components have been widely used in many fields, applications in different fields will have different requirements for the performance indicators of optical fiber imaging components.
Therefore, it is necessary to carry out comprehensive performance testing and evaluation of optical fiber imaging components to be suitable for different application backgrounds.
1. Vacuum tightness
end is placed in a vacuum environment, and the other end is placed in an atmospheric environment. If the component leaks, it will cause damage to the device and make it inoperable. Vacuum tightness is the primary performance requirement for optical fiber imaging components. The general specification requires that the air tightness should not be greater than 2×10-12Pa⋅m3/s (test conditions vary in different standards).
2. Light transmission performance
Collimated light and Lambertian light (diffuse light) are incident.
Collimated light transmittance: the ratio of the luminous flux of the output end face to the luminous flux of the input end face when the collimated light is vertically incident on the optical fiber imaging element.
Lambertian light transmittance: When Lambertian light is incident on the optical fiber image sensor, the ratio of the luminous flux at the output end face to the incident luminous flux. Light transmittance is an important performance of optical fiber imaging components, and its value directly affects the device’s screen effect, viewing distance, and cathode sensitivity. The pursuit of high light transmittance has always been the goal of the preparation of optical fiber imaging components.
3. Imaging performance
a) Spots (chicken wire):
locally bright or dark areas that are visually distinguishable. Speckle is an inherent defect of optical fiber imaging components and is a manifestation of fixed pattern noise. In the existing national standard system, it is not clearly stipulated how much the difference between light and dark relative to the background is considered as a spot.
If the relative transmittance is lower than 70% of the background, it is regarded as a speckle. For speckle requirements, the smaller the scale, the better.
In recent years, the requirements have been continuously improved, and spots larger than 90µm are not allowed to exist in optical fiber imaging components.
b) Image distortion:
generally divided into shear distortion and snake distortion. Shear distortion is the displacement or rotation of the local short line segment produced by the optical fiber imaging element to the transmitted linear pattern. The serpentine distortion is the displacement of the curve figure and the straight line generated when the optical fiber imaging element transmits the straight line pattern at the center of the incident end face.
At present, the national standard and the national military standard stipulate its values as follows: the shear distortion of the optical fiber image inverter is not greater than 30µm, and the serpentine distortion is not greater than 40µm; the shear distortion of the optical fiber panel is not greater than 30µm, and the serpentine distortion is not greater than 20µm. The shear distortion of the optical fiber cone is not more than 30 or 50µm, and the serpentine distortion is not more than 0.5% or 1% of the effective diameter (different sizes have different standards).
c) Image displacement:
The overall maximum displacement of the pattern observed from the output end face relative to the central reference point, based on the cross point in the incident end face of the optical fiber imaging element. This offset is caused by the misalignment of the optical fibers inside the optical fiber imaging element.
The current standard requires that the image displacement should not be greater than 125µm.
d) Knife edge response (imaging contrast):
Knife edge response is an important method to evaluate the quality of imaging in the optical fiber imaging element, mainly refers to the attenuation of light intensity in the shadow of the knife edge transmitted by the optical fiber imaging element.
It is stipulated in the standard that the knife edge response value of the third point position, namely 50µm, is not greater than 4.2%. However, there is a more direct method to characterize the attenuation of light intensity at the knife-edge position, which is also a method commonly used abroad, that is, to measure the relative transmittance of crosstalk light between fibers. This indicator has not been incorporated into the domestic standard system, but it is generally stipulated that the cross-light rate between optical fibers at the center position should not be greater than 2% in the internal control indicators of enterprises. For different usage requirements, the cross-beam rate between the fibers will be different.
This defect mainly exists at the secondary multifilament boundary. A continuous or semi-continuous grid pattern with obvious brightness enhancement on the boundary and a width of not more than two unit filaments is called a bright grid; on the boundary, there is a marked decrease in brightness and a width of no more than two cells A continuous or semi-continuous grid pattern of filaments, called a dark grid.
Standard requirements: 10x magnification under Lambertian light incidence without grid.
Table 1 Main characteristic parameters of optical fiber imaging components
|Single fiber diameter (um)||⌀3~6|
|Vacuum tightness (Pa m^3/s)||< 10^-12|
|Numerical aperture||≥ 1.0|
|Collimated light perspective ratio||> 65%|
|Lambertian perspective ratio||> 58%|
|Spectral range (nm)||300~2000|
|Shear distortion (um)||≤ 30|
|Serpentine Distortion (um)||≤ 40|
|Image displacement (um)||≤ 125|
|Contrast (um)||≤ 2%|
|Thermal expansion coefficient (10^7/°C)||87 ± 5|
|In the above imaging performance, the inherent imaging light and dark non-uniformity of the optical fiber imaging element is called fixed pattern noise, which mainly includes spots (line-shaped spots are called chicken wires) and grids.|
4. Thermal properties
Mainly refers to thermal expansion coefficient, heat resistance and thermal stability performance, such performance directly determines
The matching between the optical fiber imaging element and other materials is determined, as well as its own adaptability to the environment. What determines the thermal performance is the optical glass material used and the degree of matching between them.
At present, the national military standard stipulates:
20～300℃ average thermal expansion coefficient (87±5)×10-7/℃; can withstand baking at 580℃, optical properties cannot be affected when it drops to room temperature; can withstand not less than 25℃ when quenched in air The temperature difference is quenched in water, and it can withstand a temperature difference of not less than 40 °C without cracks in appearance