reflected.
An optical fibre taper (see figure 4.13b) is a spatially coherent fibre optic bundle made from conical fibres. The term coherent is used to specify that the fibres are arranged so that their terminations occupy the same relative positions in both ends of the bundle; as a consequence such kind of arrangement is able to transmit images. A property of conical fibres is that the numerical aperture at the larger end, NA^, is always smaller than the numerical aperture at the smaller end, NA^. Therefore, the effective numerical aperture of the fibre optic taper used in the demagnifying orientation is given by
AM, (4.20)
4 4
The end face of a fibre taper consists of a group of individual optical fibres closely packed together. Of all the light incident on the surface of the taper only the light that falls onto the taper cores is transmitted. The core packing fraction is defined as the fraction of the cross sectional area of the taper which is occupied by the fibre cores and is given by
(4.21) where is the core diameter, is the total fibre diameter (including any coating), and kp is a constant that depends on the packing method (see figure 4.14). For square- packed fibres = 0.785, while for hexagonally packed fibres k^ = 0.907. Typical core packing ratios range between 0.5 and 0.8.
The total coupling efficiency of a fibre optic taper depends on its effective numerical aperture, the core packing fraction and any other losses that can occur while the light is transmitted through the fibre, such as reflection losses at interfaces and absorption on the core material; typical transmission efficiencies are of the order of 0.9. Combining equations 4.21 and 4.22, the total optical energy coupling efficiency of a fibre optic taper under Lambertian illumination (Liu et al 1993) is given by
T|/„ = TlfTlr =
Chapter 4 The Charge Coupled Device
For exam ple, a 2x reducing fibre optic taper with a packing fraction rip = 0 .8 , transm ission efficiency Pp = 0 .9 and a principal num erical aperture TVA, = 1 gives an optical coupling efficiency o f -1 8 % . Therefore, fibre-optic form at alterations are a m uch m ore efficient coupling m ethod when com pared to a lens system that provides the sam e dem agnification ratio (see section 4.5.5.1).
D,
Sc^uare Hexagonal
Figure 4.14. Packing methods in typical fibre-optic bundles.
A source of contrast degradation in fibre-optically coupled devices is the presence of stray light propagating within the taper. The reduction in im age contrast when stray light is present is proportional to the square o f the effective num erical aperture and to
the packing fraction o f the taper (Liu et al 1993). If C represents the original contrast,
then the contrast at the output o f the taper w ould be
C, = N A f r \ , C (4.23) Since the principal num erical aperture (TVA^J is norm ally designed to be unity in m odern fibre-optic tapers, it can be seen from equation 4.22 that contrast degradation is inversely proportional to the square of the taper ratio.
The m ost com m on technique used to reduce the effect o f stray light is to include a
highly absorbing m aterial betw een the individual fibres, w hich is called extra m ural
absorber (EM A ). H ow ever, a com prom ise m ust be reached betw een the thickness of the EM A , w hich reduces the effective packing fraction o f the taper, and the degree of absorption required.
Chapter 4 The Charge Coupled Device
4.6. Radiation damage
4.6.1. Radiation damage to the CCD
A significant amount of radiation can permanently damage the performance of a CCD. Different performance parameters are affected in different ways by various types of ionising radiation, but in general terms it has been observed that thermal carrier generation and readout noise increase, while charge transfer efficiency decreases. There are essentially two types of radiation damage, those related to ionisation and those caused by displacement in the crystalline lattice of the silicon. The latter is only related to high energy ionising particles. This means that the maximum dose of a particular kind of radiation that can be tolerated by a CCD will depend on nature of the application. As a general guidance for the case of ionising radiation it has been observed that if the dose in the silicon is less that -10^ Gy the performance parameters remain practically unchanged; at -10^ Gy a significant change can be observed and finally above-10* Gy there is serious degradation of performance (EEV 1987).
Degradation of performance normally occurs slowly as the dose builds up and very rarely the effects are catastrophic. For example, studies carried out by EEV indicate that dark current at room temperature increases at a rate of approximately 1 nA/cm^ per 10^ Gy. In comparison, a typical peak signal current level is of the order of 50 nA/cm^. Some of the damage can be repaired and the original characteristics partially restored using annealing procedures. However, radiation hardness is normally reduced after annealing.
4.6.2. Radiation damage to the phosphor coating
In general terms doped alkali halides such as CsI(Tl) are also susceptible to radiation damage. Prolonged exposure or intense radiation can change their scintillation characteristics. The damage normally manifests itself as a gradual decrease of the relative signal size. Measurements of light output in evaporated CsI(Tl) layers of the type used in image intensifier tubes indicate a reduction of approximately 30% in signal size after an accumulated dose of -10^ Gy (Perez-Mendez 1990). Although some radiation damage can be noticeable above 10 Gy, most of the degradation occurs on the optical absorption bands at low wavelengths and therefore it does not produce a very noticeable effect when used with photodiodes or CCDs.