The optical coupling of intensified CCD cameras
The design of ICCD cameras make it necessary to optical couple the output image of the image intensifier to the CCD sensor. Two technical approaches are competing with one another for the best optical coupling solution: the fiber coupling and lens coupling. All ICCD cameras from Stanford Computer Optics, Inc are equipped with a in-house developed, customized F/0.8 lens which provides best image quality without vignetting, distortion or honeycomb pattern.
The coupling lens provides the optical coupling of the image intensifier with the CCD sensor.
Specifications of the perfect optical coupling
The perfect optical coupling have two specifications: image quality and coupling efficiency. The image need to coupled preserving the optical resolution, without introducing any optical aberration like distortion or vignetting, neither is a systematic image error like honeycomb pattern tolerable. The perfect coupling efficiency is 100% which means all emitted photons reach the CCD sensor.
Competing coupling methods: Coupling Lens and Fiber Taper
The phosphor screen of the image intensifieremits the intensified object image as a Lambertian radiator. The lens coupling is using a number of lenses to image the phosphor screen to the CCD sensor. The fiber optical coupling uses a bunch of optical fibers which are clued on the one site to the phosphor screen and with the second end to the CCD sensor.
A summary of the advantages and disadvantages of optical lens coupling and fiber optical coupling:
| Lens Coupling | Fiber taper | |
| Advantage | + excellent coupling efficiency by inhouse developed F/0.8 lens + superior image quality
+ variable setup (e.g. easy repair and replacement ofe ach single component, especially image intensifier) |
+ good coupling efficiency |
| Disadvantage | - stretched design | - poor image quality
- fixed structure e.g. no repair or replacement |
A comparison of the coupling efficiency
The coupling efficiency of fiber optical coupling is by far not 100% which is a very common misbelief. Due to the technical conditions the light coupling into any optical fiber has the constrain that the angle of the light must be less than the numeric aperture (NA) of the optical fiber. Furthermore, each fiber consists of a cladding and a core and imaging light is only transferred in core and blocked in cladding. A typical number of the core/cladding ratio fiber optic tapers is 50/50 [1] and of fiber optic faceplates 70/30 [2]. If we assume the phosphor screen of the image intensifiers as a homogenous emitting Lambertian radiator which is placed directly on the fiber entrance. Then only imaging light which is emitted directly on the fiber core and with an angle less than the NA of the optical fiber is transferred to the CCD sensor. Additionally, the transmission of white light in fiber optical tapers is given with 70% per 10mm [2, 3] which significantly reduces the signal at the CCD sensor.
In comparison the optical lens coupling has no area loss and the total coupling efficiency is given by the F-number. Furthermore, the lens coupling provides a higher transmission of coupled light. Therefore, the in-house developed F/0.8 coupling lens has no disadvantage in comparison to fiber optical coupling in terms of coupling efficiency.
Measurement of the coupling efficiency
The above mentioned theoretical considerations are verified by B. J. Patrie, et al. from the Stanford University [4]. They measured the fiber optic coupling efficiency on a experimental setup and concluded that the fiber optic coupling has a efficiency of approximately 1.0%. They compared the fiber optic coupling to a commercially available F/2.0 lens coupling which has a coupling efficiency of approximately 0.4%.
A comparison of the coupled image quality
The image quality is crucial for many imaging applications of intensified CCD cameras. The Modulation transfer function (MTF) is often used as a standard performance evaluation for optical components and imaging systems. The lens quality is defined by its MTF function, however, it is difficult to evaluate the MTF of fiber optic tapers since these coupling elements violate the isoplanatic condition of the MTF evaluation [5, 6]. Although there were some research works with the MTF measuring methods and results for evaluating the quality of output image from the fiber optical plate [7]. The cut of frequency of the MTF is most commonly declared as maximum resolution. Moreover, the MTF provides no information about systematic image aberrations like optical distortion and vignetting which diminish the image quality and should be avoided, as far as possible.
MTF comparison of lens and fiber optic coupling
The following diagrams show a comparison of the Modulation Transfer Functions (MTF) of the particular designed coupling lenses and commercially available photo lens. The first diagram gives the MTF as a function of the spatial frequency. The solid curves show the MTF at several radial positions of the images cross section, ranging from the center at r = 0mm to the brink at r = R, with R the radius of the image. It can easily be seen that the MTF is almost independent of the radial position on the image and even at a spatial frequency of 200 lp/mm the MTF still amounts to 30% and thereby lies far above the optical resolution limit of 3%.
For comparison, the first diagram also includes dashed MTF curves for a commercially available aspherical photo lens with a focal length of 90mm. As known from photo lenses, the MTF strongly decreases with both the number of linepairs and also with the image radius although the lens was stopped down to f8. Because the image intensifier is the most serious resolution limiting component of an ICCD camera it is essential that the coupling optics keeps the image intensifiers optical resolution. However, a photo lens coupling, and also a tapered fiber bundle, would significantly degrade the attainable image resolution.
The hatched area above the diffraction limit represents the physical limit of the MTF due to the finite diameter of the lenses. To keep clearness in the diagrams, only averaged curves including both sagittal and tangential image structures are plotted.
Considerations of image aberrations and geometrical dislocations
The in-house developed F/0.8 coupling lens from Stanford Computer Optics provides a excellent uniform image without vignetting and no image distortion (< 0.03%). In contrast the fiber optic coupling has in general a uniformity across the image area of only 3% [8]. Additionally, the fiber optic bundle pattern ("chicken wire") caused by opaque fibers at bundle interfaces is frequently seen. Various kinds of image errors like barrel or pincushion distortion, geometrical shear or skew distortion and single-point blemishes have been observed with fiber optic coupling [8]. Considering the image quality the lens coupling has tremendous advantages as against the fiber optic coupling.
System design considerations of lens and fiber optical coupling
Due to the fact that optical fiber coupling is mechanically glued to the image intensifier and the CCD sensor this configuration is stable and compact. However, in case of any defect neither of the components can be replaced. In comparison the lens coupling does provide a flexible system design and enables the replacement of individual components of the ICCD cameras. Moreover, the lens coupling enables the coupling of image intensifiers with almost any available camera. This concept is realized by the stand alone image intensifier module, Quantum Leap.
Coupling Lens have prevailing advantages vs. fiber optical coupling
In reconsideration of all above discussed arguments of the hereby compared optical coupling methods, the lens coupling have prevailing advantages in comparison to the fiber optical coupling. The advantages are very clear in terms of image quality and system flexibility and the most commonly assumed lack in coupling efficiency is almost identical. Due to these reasons, all ICCD cameras from Stanford Computer Optics, Inc are equipped with the in-house developed F/0.8 coupling lens for excellent coupling efficiency and superior image quality.
References:
1) Edmund Optics: Fiber Optic Tapers and Faceplates, http://www.edmundoptics.com/optics/fiber-optics/fiber-optic-tapers-faceplates/1599 (13.06.2014)
2) Schott AG: Fused Imaging Fiber Optic Tapers, http://www.schott.com/lightingimaging/english/download/tapers.pdf (13.06.2014)
3) Schott AG: Schott Fiber Optic Faceplates, http://www.schott.com/lightingimaging/english/download/schott-faceplate_int_july_2013.pdf (13.06.2014)
4) Bryan J. Patrie, Jerry M. Seitzman and Ronald K. Hanson, "Instantaneous three-dimensional flow visualization by rapid acquisition of multiple planar flow images", Opt. Eng. 33(3), 975-980 (Mar 01, 1994)
5) T. Carter, " Sampled MTF of fused fiber optic components and bonded assemblies", Proc. SPIE 8735, Head- and Helmet-Mounted Displays XVIII: Design and Applications, 873507 (May 16, 2013);
6) Yaoxiang Wang, Weijian Tian and XiangLi Bin, "Theoretical model of the modulation transfer function for fiber optic taper", Proc. SPIE 5638, Optical Design and Testing II, 865 (April 06, 2005)
7) Weijian Tian, Wei Zhang ; Weihong Ma ; Yaoxiang Wang and Yulin Li, "Imaging Performance Evaluation for Fiber Optical Plates with Modulation Transfer Function", Proc. SPIE 4927, Optical Design and Testing, 193 (September 19, 2002)
8) C.I. Coleman, Imaging Characteristics of Rigid Coherent Fiber Optic Tapers, Advances in Electronics and Electron Physics Volume 64, Part B, 1985, Pages 649–661