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CCD Detector
ADVANCES IN ELECTRONICS A N D EI.ETTKON PHYSICS. VOL. M A
LLL TV Imaging with GaAs PhotocathodeKCD Detector Y. BEAUVAIS, J . CHAUTEMPS, and P. DE GROOT
INTRODUCTION The present article deals with a low-light level (LLL) imaging detector consisting of a third-generation image intensifier which is optically coupled to a solid-state CCD array. This device combines the advantages of both components: the high sensitivity and low-dark current of the GaAs photocathode, and the small dimensions, the solidity, the reliability, and the electrical performance of solid-state CCD arrays used as T V imaging analyzers. GaAs photocathodes are very well suited for imaging with low-level natural illumination, due to their high and constant quantum efficiency (typically 25%) in the 0.6- to 0.9-pm spectral range. Although the sensitivity of the GaAs photocathode is considerably higher than that of conventional multialkaline photocathodes (Fig. I ) , it still remains lower by a factor of two than the internal sensitivity of the CCD device itself. In the CCD the photoelectrons are not emitted into vacuum. However, the use of direct CCD imaging at low-light levels is severely limited by the important thermal noise fluctuations which occur in silicon at ambient temperatures. Typically, for the TH 7861 CCD frame transfer matrix with a pixel size of 23 x 23 pm?, the thermal noise fluctuations correspond to about 120 electrons per pixel per 20 msec, which is about 4 orders of magnitude higher than the thermal noise of the GaAs photocathode (typically A cm-? or less). Hence a high-performance quantum noise limited LLL imaging device can be realized by coupling a GaAs photocathode with a CCD matrix, provided that an intermediate electron gain of about lo4 is obtained.
ELECTRON GAIN Electron gain can be obtained either directly, by electron bombardment of the CCD, or by optical coupling with an image intensifier. In the second 267 Copyright
(17 198.5 by Academic Prer\, Inu. (L.ondon) L.rd. All rights of reproduction in any form re\erved. ISBN 0-12-014664-9
268
7 - - - r1 Y. BEAUVAIS E T A L .
0.6
-z
0.5
1
0
i
I-
0
(TH 78611
0.4
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z 0 a t-
-
0.3
-I
w n
1
w>
0.2
5tz
$
0.1
0
0.4
0.5
0.6
07
0.8
0.9
1
WAVELENGTH (urn)
FIG.1. Spectral response of some photocathodes compared with the spectral sensitivity of the TH 7861 CCD matrix.
case the gain is mainly obtained by the use of an electron multiplier such as a microchannel plate.
Electron Bombardment High-voltage acceleration of the photoelectrons through the vacuum toward the CCD provides electron gain by creating secondaries in the silicon wafer and provides simultaneously electron optical focusing. However, a back-thinned CCD is required. The achievable gain corresponds to a creation of one electron-hole pair by each 3.6 eV of incident energy above the dead layer voltage (which is in the keV range). Typically an accelerating voltage exceeding 15 keV is necessary to obtain a gain of lo4. This voltage makes assembly difficult if proximity focusing is to be used between a GaAs photocathode and a thinned CCD array. Such detectors have not yet been demonstrated, however this approach is still attractive as it minimizes resolution losses.
Optical Coupling ~ G t i un i Image Intensifier
Very high gains can be achieved in this way by combining the gain of the image intensifier itself, good spectral matching, and demagnifying coupling optics. By reference to currently measured characteristics of each element, the sensitivity of the assembly can be expressed as SlW= VCCD E W
-
T S, T S,Ga - - - G a 7 Sw in2-
SW
m-
=
gSw
where Vccll is the output signal of the CCD corresponding to white light illumination, EW , of the photocathode; SW and S, are the sensitivities of the CCD to white light and to the light of the phosphor screen of the image intensifier; G is the luminous gain of the image intensifier, (Y characterizes the angular distribution of the phosphor light ( a = 7~ if lambertian), and T and m are the efficiency and the magnification of the optical coupling. Reference has been made to white light sensitivities because of their common use. The above expression shows that an electron gain of lo4 can be obtained using an image intensifier with a luminous gain of 9000, and a direct fiber optic coupling with a demagnification of 0.5, (S, = 10 mV lux-', P - 2 0 ; Sw = 18 mV lux-'; a = 2.2; T = 0.25). This demagnification matches the useable areas of the image intensifier and the CCD in the detector that we have assembled and tested.
Low LIGHTLEVELDETECTOR ASSEMBLY Figure 2 shows the assembly which comprises three main parts: the image intensifier, the fiber optics, and the CCD. The Third-Generation Imcige Intensifier THX I314
The GaAs photocathode is bonded onto a thick glass window. The P . 2 0 phosphor screen is deposited onto plane fiber optics. Proximity focusing with reduced gaps between the photocathode and the microchannel plate, and between the microchannel plate and the screen, provides resolution better than 25 Ip mm--'. The microchannel plates are filmed at the input surface and have funnelled channel entrances. Tapered Fiber Optics
The fiber optics are shaped to fit the CCD surface. Optical coupling to the CCD is obtained with a layer of immersion oil whose thickness is
270
Y. BEAUVAIS E T A L . MICROCHANNEL PLATE I
TAPERED t l B E R OPTICS
PHO
limited to less than 10 pm by mechanical pressing. The tapered fiber optics and the CCD are first assembled and bonded to a metal wafer and then tested. Coupling with the image intensifier is performed using the wafer as a mechanical reference. The CCD Matrix The CCD matrix TH 7861 is of the frame transfer type offering 576 lines of 384 elements. This image format corresponds to the standard % in. The spectral response and sensitivity of this device are given in Fig. I . The high voltage for the image intensifier is supplied by a miniature voltage multiplier providing automatic gain control. This power supply is situated beside the detector in the test camera. The overall dimensions of the potted device are 45 mm in diameter and 41 mm in length.
\
8 t
01
k
!i
8
00I
10-5
104
10 PHOTOCATHODE
10-1
102
lL LUMINATION
I
(LUX1
FIG.3. Output signal versus photocathode illumination of the assembly.
PERFORMANCE OF T H E
ASSEMBLY
Sensitivity and Dynamic Range
Figure 3 shows the dependance of CCD output on photocathode illumination. The low-light level sensitivity is 140 V lux-', which is about lo4 times that of the CCD alone. The plateau of the curve corresponds to the screen luminance limitation by the automatic gain control from the power supply when the photocathode is uniformly illuminated. For practical pictures the luminance of the brightest parts is limited only by the saturation of the microchannel plate at a value at least four times greater than those corresponding to the plateau value, so that the whole dynamic range of the CCD is preserved. Resolution and MTF
The main limitation in cascading the elements of the assembly results from the resolution losses. The MTF of each element and the computed MTF of the assembly are shown in Fig. 4. The MTF of the fiber optic coupling has been deduced from previous measurements with similar couplings. Direct measurements of the MTF of the device are in good
10
Y. BEAUVAlS ET A L .
+I
Y-
I 1-, 1 NYQUIST FREQUENCY
10
SPATIAL FREQUENCY (CDD PLAN
FIG.4. MTF curves of the separate elements and of the detector assembly. a, F.O. coupling; b , TH 7861; c, THX 1314 (after F.O. demagnification); d, THX 35 106 (LLL TV assembly).
10-6
10-5
10-4
10-3
10-2
PHOTOCATHODE ILLUMINATION (LUX)
FIG.5. Low-light level limiting resolution.
273
FIG.6. Picture taken with 10
lux of photocathode illumination.
agreement with the computed values. At the lowest levels of illumination, the resolution is limited by photon noise, and the signal-to-noise ratio can be related to photocathode sensitivity and to the noise figure of the microchannel plate. Figure 5 shows the low-light level limiting resolution. Figure 6 shows a picture taken at lux photocathode illumination as displayed on the TV monitor. One of the most striking features of this picture is the absence of distortion.
CONCLUSION The LLL TV detector described here displays the two main advantages of this type of optical coupling assemblies: ( I ) each element of the device can be selected and optimized for its specific application; and (2) the technology is simpler than that of a more sophisticated integrated device, thus leading to lower costs. The low-light level imaging performance can be compared to that of the much more cumbersome TV pick-up tubes. Improvement of the limiting resolution at high illuminations is desirable in order to take advantage of the wide dynamic range of the detector.
274
Y. BEAUVAIS ET A L .
Some improvements can still be expected in the MTF of the image intensifier by reducing the pitch of the microchannel plate, by using an intagliated phosphor screen, and from a reduction of the losses of resolution in the optical coupling. Furthermore, the use of components with larger useful areas and similar intrinsic resolution will increase the overall performance while retaining the small dimensions and low weight of the detector.
LLL TV Imaging with GaAs PhotocathodeKCD Detector Y. BEAUVAIS, J . CHAUTEMPS, and P. DE GROOT
INTRODUCTION The present article deals with a low-light level (LLL) imaging detector consisting of a third-generation image intensifier which is optically coupled to a solid-state CCD array. This device combines the advantages of both components: the high sensitivity and low-dark current of the GaAs photocathode, and the small dimensions, the solidity, the reliability, and the electrical performance of solid-state CCD arrays used as T V imaging analyzers. GaAs photocathodes are very well suited for imaging with low-level natural illumination, due to their high and constant quantum efficiency (typically 25%) in the 0.6- to 0.9-pm spectral range. Although the sensitivity of the GaAs photocathode is considerably higher than that of conventional multialkaline photocathodes (Fig. I ) , it still remains lower by a factor of two than the internal sensitivity of the CCD device itself. In the CCD the photoelectrons are not emitted into vacuum. However, the use of direct CCD imaging at low-light levels is severely limited by the important thermal noise fluctuations which occur in silicon at ambient temperatures. Typically, for the TH 7861 CCD frame transfer matrix with a pixel size of 23 x 23 pm?, the thermal noise fluctuations correspond to about 120 electrons per pixel per 20 msec, which is about 4 orders of magnitude higher than the thermal noise of the GaAs photocathode (typically A cm-? or less). Hence a high-performance quantum noise limited LLL imaging device can be realized by coupling a GaAs photocathode with a CCD matrix, provided that an intermediate electron gain of about lo4 is obtained.
ELECTRON GAIN Electron gain can be obtained either directly, by electron bombardment of the CCD, or by optical coupling with an image intensifier. In the second 267 Copyright
(17 198.5 by Academic Prer\, Inu. (L.ondon) L.rd. All rights of reproduction in any form re\erved. ISBN 0-12-014664-9
268
7 - - - r1 Y. BEAUVAIS E T A L .
0.6
-z
0.5
1
0
i
I-
0
(TH 78611
0.4
\
z 0 a t-
-
0.3
-I
w n
1
w>
0.2
5tz
$
0.1
0
0.4
0.5
0.6
07
0.8
0.9
1
WAVELENGTH (urn)
FIG.1. Spectral response of some photocathodes compared with the spectral sensitivity of the TH 7861 CCD matrix.
case the gain is mainly obtained by the use of an electron multiplier such as a microchannel plate.
Electron Bombardment High-voltage acceleration of the photoelectrons through the vacuum toward the CCD provides electron gain by creating secondaries in the silicon wafer and provides simultaneously electron optical focusing. However, a back-thinned CCD is required. The achievable gain corresponds to a creation of one electron-hole pair by each 3.6 eV of incident energy above the dead layer voltage (which is in the keV range). Typically an accelerating voltage exceeding 15 keV is necessary to obtain a gain of lo4. This voltage makes assembly difficult if proximity focusing is to be used between a GaAs photocathode and a thinned CCD array. Such detectors have not yet been demonstrated, however this approach is still attractive as it minimizes resolution losses.
Optical Coupling ~ G t i un i Image Intensifier
Very high gains can be achieved in this way by combining the gain of the image intensifier itself, good spectral matching, and demagnifying coupling optics. By reference to currently measured characteristics of each element, the sensitivity of the assembly can be expressed as SlW= VCCD E W
-
T S, T S,Ga - - - G a 7 Sw in2-
SW
m-
=
gSw
where Vccll is the output signal of the CCD corresponding to white light illumination, EW , of the photocathode; SW and S, are the sensitivities of the CCD to white light and to the light of the phosphor screen of the image intensifier; G is the luminous gain of the image intensifier, (Y characterizes the angular distribution of the phosphor light ( a = 7~ if lambertian), and T and m are the efficiency and the magnification of the optical coupling. Reference has been made to white light sensitivities because of their common use. The above expression shows that an electron gain of lo4 can be obtained using an image intensifier with a luminous gain of 9000, and a direct fiber optic coupling with a demagnification of 0.5, (S, = 10 mV lux-', P - 2 0 ; Sw = 18 mV lux-'; a = 2.2; T = 0.25). This demagnification matches the useable areas of the image intensifier and the CCD in the detector that we have assembled and tested.
Low LIGHTLEVELDETECTOR ASSEMBLY Figure 2 shows the assembly which comprises three main parts: the image intensifier, the fiber optics, and the CCD. The Third-Generation Imcige Intensifier THX I314
The GaAs photocathode is bonded onto a thick glass window. The P . 2 0 phosphor screen is deposited onto plane fiber optics. Proximity focusing with reduced gaps between the photocathode and the microchannel plate, and between the microchannel plate and the screen, provides resolution better than 25 Ip mm--'. The microchannel plates are filmed at the input surface and have funnelled channel entrances. Tapered Fiber Optics
The fiber optics are shaped to fit the CCD surface. Optical coupling to the CCD is obtained with a layer of immersion oil whose thickness is
270
Y. BEAUVAIS E T A L . MICROCHANNEL PLATE I
TAPERED t l B E R OPTICS
PHO
limited to less than 10 pm by mechanical pressing. The tapered fiber optics and the CCD are first assembled and bonded to a metal wafer and then tested. Coupling with the image intensifier is performed using the wafer as a mechanical reference. The CCD Matrix The CCD matrix TH 7861 is of the frame transfer type offering 576 lines of 384 elements. This image format corresponds to the standard % in. The spectral response and sensitivity of this device are given in Fig. I . The high voltage for the image intensifier is supplied by a miniature voltage multiplier providing automatic gain control. This power supply is situated beside the detector in the test camera. The overall dimensions of the potted device are 45 mm in diameter and 41 mm in length.
\
8 t
01
k
!i
8
00I
10-5
104
10 PHOTOCATHODE
10-1
102
lL LUMINATION
I
(LUX1
FIG.3. Output signal versus photocathode illumination of the assembly.
PERFORMANCE OF T H E
ASSEMBLY
Sensitivity and Dynamic Range
Figure 3 shows the dependance of CCD output on photocathode illumination. The low-light level sensitivity is 140 V lux-', which is about lo4 times that of the CCD alone. The plateau of the curve corresponds to the screen luminance limitation by the automatic gain control from the power supply when the photocathode is uniformly illuminated. For practical pictures the luminance of the brightest parts is limited only by the saturation of the microchannel plate at a value at least four times greater than those corresponding to the plateau value, so that the whole dynamic range of the CCD is preserved. Resolution and MTF
The main limitation in cascading the elements of the assembly results from the resolution losses. The MTF of each element and the computed MTF of the assembly are shown in Fig. 4. The MTF of the fiber optic coupling has been deduced from previous measurements with similar couplings. Direct measurements of the MTF of the device are in good
10
Y. BEAUVAlS ET A L .
+I
Y-
I 1-, 1 NYQUIST FREQUENCY
10
SPATIAL FREQUENCY (CDD PLAN
FIG.4. MTF curves of the separate elements and of the detector assembly. a, F.O. coupling; b , TH 7861; c, THX 1314 (after F.O. demagnification); d, THX 35 106 (LLL TV assembly).
10-6
10-5
10-4
10-3
10-2
PHOTOCATHODE ILLUMINATION (LUX)
FIG.5. Low-light level limiting resolution.
273
FIG.6. Picture taken with 10
lux of photocathode illumination.
agreement with the computed values. At the lowest levels of illumination, the resolution is limited by photon noise, and the signal-to-noise ratio can be related to photocathode sensitivity and to the noise figure of the microchannel plate. Figure 5 shows the low-light level limiting resolution. Figure 6 shows a picture taken at lux photocathode illumination as displayed on the TV monitor. One of the most striking features of this picture is the absence of distortion.
CONCLUSION The LLL TV detector described here displays the two main advantages of this type of optical coupling assemblies: ( I ) each element of the device can be selected and optimized for its specific application; and (2) the technology is simpler than that of a more sophisticated integrated device, thus leading to lower costs. The low-light level imaging performance can be compared to that of the much more cumbersome TV pick-up tubes. Improvement of the limiting resolution at high illuminations is desirable in order to take advantage of the wide dynamic range of the detector.
274
Y. BEAUVAIS ET A L .
Some improvements can still be expected in the MTF of the image intensifier by reducing the pitch of the microchannel plate, by using an intagliated phosphor screen, and from a reduction of the losses of resolution in the optical coupling. Furthermore, the use of components with larger useful areas and similar intrinsic resolution will increase the overall performance while retaining the small dimensions and low weight of the detector.